diff --git a/Monkey/Lexer/Lexer.lean b/Monkey/Lexer/Lexer.lean
index 3d09d5b..40949ab 100644
--- a/Monkey/Lexer/Lexer.lean
+++ b/Monkey/Lexer/Lexer.lean
@@ -56,7 +56,7 @@ def readIdentifier : StateM Lexer String := do
     |>.drop position
 
 /-- Lexer を更新しつつ、Number ではない文字列が出てくるまで読み進める -/
-def readNumber : StateM Lexer String := do
+def readNumber : StateM Lexer Int := do
   let mut l ← get
   let position := l.position
   while l.ch.isDigit do
@@ -65,8 +65,9 @@ def readNumber : StateM Lexer String := do
   return l.input
     |>.take l.position
     |>.drop position
+    |>.toInt!
 
-open TokenType
+open Token
 
 -- Char を String に変換する関数
 #check String.singleton
@@ -83,34 +84,31 @@ def skipWhitespace : StateM Lexer Unit := do
 def nextToken : StateM Lexer Token := do
   skipWhitespace
   let mut l ← get
-  let ch := String.singleton l.ch
   let mut tok := match l.ch with
-    | '=' => ⟨.ASSIGN, ch⟩
-    | '+' => ⟨.PLUS, ch⟩
-    | '-' => ⟨.MINUS, ch⟩
-    | '!' => ⟨.BANG, ch⟩
-    | '/' => ⟨.SLASH, ch⟩
-    | '*' => ⟨.ASTERISK, ch⟩
-    | '<' => ⟨.LT, ch⟩
-    | '>' => ⟨.GT, ch⟩
-    | '(' => ⟨.LPAREN, ch⟩
-    | ')' => ⟨.RPAREN, ch⟩
-    | '{' => ⟨.LBRACE, ch⟩
-    | '}' => ⟨.RBRACE, ch⟩
-    | ',' => ⟨.COMMA, ch⟩
-    | ';' => ⟨.SEMICOLON, ch⟩
-    | '\x00' => ⟨.EOF, ""⟩
-    | _ => ⟨.ILLEGAL, ch⟩
+    | '=' => ASSIGN
+    | '+' => PLUS
+    | '-' => MINUS
+    | '!' => BANG
+    | '/' => SLASH
+    | '*' => ASTERISK
+    | '<' => LT
+    | '>' => GT
+    | '(' => LPAREN
+    | ')' => RPAREN
+    | '{' => LBRACE
+    | '}' => RBRACE
+    | ',' => COMMA
+    | ';' => SEMICOLON
+    | '\x00' => EOF
+    | _ => ILLEGAL
   if l.ch.isLetter then
     let literal ← readIdentifier
-    let tokenType := LookupIdent literal
-    tok := ⟨tokenType, literal⟩
-    return tok
+    let token := LookupIdent literal
+    return token
   else if l.ch.isDigit then
     let literal ← readNumber
-    let tokenType := INT
-    tok := ⟨tokenType, literal⟩
-    return tok
+    let token : Token := INT literal
+    return token
   readChar
   return tok
 
diff --git a/Monkey/Lexer/LexerTest.lean b/Monkey/Lexer/LexerTest.lean
index 6a36c16..511b7bf 100644
--- a/Monkey/Lexer/LexerTest.lean
+++ b/Monkey/Lexer/LexerTest.lean
@@ -1,33 +1,32 @@
 import Monkey.Lexer.Lexer
 import Monkey.Token.Token
 
-open TokenType Lexer
+open Token Lexer
 
 /-- NestToken 関数をテストする -/
-def testNextToken (input : String) (expected : Array (TokenType × String)) : IO Unit := do
+def testNextToken (input : String) (expected : Array Token) : IO Unit := do
   let mut l : Lexer := Lexer.new input
   for tt in expected do
+  for expTok in expected do
     let ⟨tok, l'⟩ := l.nextToken |>.run
     l := l'
-    if tok.type ≠ tt.fst then
-      throw <| .userError s!"tests failed: - tokentype wrong at \"{tt.snd}\". expected={tt.fst}, got={tok.type}"
-    if tok.literal ≠ tt.snd then
-      throw <| .userError s!"tests failed: - literal wrong. expected={tt.snd}, got={tok.literal}"
+    if tok ≠ expTok then
+      throw <| .userError s!"tests failed: - token wrong at \"{expTok}\". expected={expTok}, got={tok}"
 
   IO.println s!"ok!"
 
 #eval testNextToken
   (input := "=+(){},;")
   (expected := #[
-    (ASSIGN, "="),
-    (PLUS, "+"),
-    (LPAREN, "("),
-    (RPAREN, ")"),
-    (LBRACE, "{"),
-    (RBRACE, "}"),
-    (COMMA, ","),
-    (SEMICOLON, ";"),
-    (EOF, "")
+    ASSIGN,
+    PLUS,
+    LPAREN,
+    RPAREN,
+    LBRACE,
+    RBRACE,
+    COMMA,
+    SEMICOLON,
+    EOF
   ])
 
 #eval testNextToken
@@ -39,58 +38,87 @@ def testNextToken (input : String) (expected : Array (TokenType × String)) : IO
   let result = add(five, ten);
   ")
   (expected := #[
-    (LET, "let"),
-    (IDENT, "five"),
-    (ASSIGN, "="),
-    (INT, "5"),
-    (SEMICOLON, ";"),
-    (LET, "let"),
-    (IDENT, "ten"),
-    (ASSIGN, "="),
-    (INT, "10"),
-    (SEMICOLON, ";"),
-    (LET, "let"),
-    (IDENT, "add"),
-    (ASSIGN, "="),
-    (FUNCTION, "fn"),
-    (LPAREN, "("),
-    (IDENT, "x"),
-    (COMMA, ","),
-    (IDENT, "y"),
-    (RPAREN, ")"),
-    (LBRACE, "{"),
-    (IDENT, "x"),
-    (PLUS, "+"),
-    (IDENT, "y"),
-    (SEMICOLON, ";"),
-    (RBRACE, "}"),
-    (SEMICOLON, ";"),
-    (LET, "let"),
-    (IDENT, "result"),
-    (ASSIGN, "="),
-    (IDENT, "add"),
-    (LPAREN, "("),
-    (IDENT, "five"),
-    (COMMA, ","),
-    (IDENT, "ten"),
-    (RPAREN, ")"),
-    (SEMICOLON, ";"),
-    (EOF, "")
+    LET,
+    IDENT "five",
+    ASSIGN,
+    INT 5,
+    SEMICOLON,
+    LET,
+    IDENT "ten",
+    ASSIGN,
+    INT 10,
+    SEMICOLON,
+    LET,
+    IDENT "add",
+    ASSIGN,
+    FUNCTION,
+    LPAREN,
+    IDENT "x",
+    COMMA,
+    IDENT "y",
+    RPAREN,
+    LBRACE,
+    IDENT "x",
+    PLUS,
+    IDENT "y",
+    SEMICOLON,
+    RBRACE,
+    SEMICOLON,
+    LET,
+    IDENT "result",
+    ASSIGN,
+    IDENT "add",
+    LPAREN,
+    IDENT "five",
+    COMMA,
+    IDENT "ten",
+    RPAREN,
+    SEMICOLON,
+    EOF
   ])
 
 #eval testNextToken
   (input := "!-/*5;
   5 < 10 > 5;")
   (expected := #[
-    (BANG, "!"),
-    (MINUS, "-"),
-    (SLASH, "/"),
-    (ASTERISK, "*"),
-    (INT, "5"),
-    (SEMICOLON, ";"),
-    (INT, "5"),
-    (LT, "<"),
-    (INT, "10"),
-    (GT, ">"),
-    (INT, "5")
+    BANG,
+    MINUS,
+    SLASH,
+    ASTERISK,
+    INT 5,
+    SEMICOLON,
+    INT 5,
+    LT,
+    INT 10,
+    GT,
+    INT 5,
+    SEMICOLON,
+    EOF
+  ])
+
+#eval testNextToken
+  (input := "if (5 < 10) {
+  return true;
+  } else {
+  return false;
+  }")
+  (expected := #[
+    IF,
+    LPAREN,
+    INT 5,
+    LT,
+    INT 10,
+    RPAREN,
+    LBRACE,
+    RETURN,
+    TRUE,
+    SEMICOLON,
+    RBRACE,
+    ELSE,
+    LBRACE,
+    RETURN,
+    FALSE,
+    SEMICOLON,
+    RBRACE,
+    EOF
   ])
diff --git a/Monkey/Token/Token.lean b/Monkey/Token/Token.lean
index dff7311..06b21d8 100644
--- a/Monkey/Token/Token.lean
+++ b/Monkey/Token/Token.lean
@@ -1,15 +1,19 @@
 import Lean.Data.HashMap
 
-/-- Token の種類 -/
-inductive TokenType where
+/-- トークン
+
+本にある Token の定義では structure になっていたので、
+literal が必要ないのときでも literal の情報があって冗長だった。
+それを修正した定義にしている。 -/
+inductive Token where
+  /-- 識別子 -/
+  | IDENT (name : String)
+  /-- 数値リテラル -/
+  | INT (value : Int)
   /-- 受け入れ不能エラー -/
   | ILLEGAL
   /-- ファイル終端 -/
   | EOF
-  /-- 識別子 -/
-  | IDENT
-  /-- 整数 -/
-  | INT
   /-- 代入記号 "=" -/
   | ASSIGN
   /-- 足し算記号 + -/
@@ -42,15 +46,25 @@ inductive TokenType where
   | LT
   /-- 大なり ">" -/
   | GT
-deriving Repr, DecidableEq
+  /-- true : Bool -/
+  | TRUE
+  /-- false : Bool -/
+  | FALSE
+  /-- IF キーワード -/
+  | IF
+  /-- ELSE キーワード -/
+  | ELSE
+  /-- RETURN キーワード -/
+  | RETURN
+deriving Repr, BEq, DecidableEq
 
-/-- TokenType を文字列に変換する -/
-def TokenType.toString (t : TokenType) : String :=
+/-- Token を文字列に変換する -/
+def Token.toString (t : Token) : String :=
   match t with
   | .ILLEGAL => "ILLEGAL"
   | .EOF => "EOF"
-  | .IDENT => "IDENT"
-  | .INT => "INT"
+  | .IDENT lit => lit
+  | .INT lit => ToString.toString lit
   | .ASSIGN => "="
   | .PLUS => "+"
   | .COMMA => ","
@@ -67,27 +81,32 @@ def TokenType.toString (t : TokenType) : String :=
   | .SLASH => "/"
   | .LT => "<"
   | .GT => ">"
+  | .TRUE => "TRUE"
+  | .FALSE => "FALSE"
+  | .IF => "IF"
+  | .ELSE => "ELSE"
+  | .RETURN => "RETURN"
 
-instance : ToString TokenType where
-  toString := TokenType.toString
-
-set_option linter.missingDocs false in
-
-/-- トークン -/
-structure Token where
-  type : TokenType
-  literal : String
-deriving Repr, BEq, DecidableEq
+instance : ToString Token where
+  toString := Token.toString
 
-open TokenType Lean
+open Lean Token
 
-/-- 言語のキーワード -/
-def keywords : HashMap String TokenType :=
-  let list : List (String × TokenType) := [("fn", FUNCTION), ("let", LET)]
+/-- 言語のキーワードを格納する辞書 -/
+def keywords : HashMap String Token :=
+  let list : List (String × Token) := [
+    ("fn", FUNCTION),
+    ("let", LET),
+    ("true", TRUE),
+    ("false", FALSE),
+    ("if", IF),
+    ("else", ELSE),
+    ("return", RETURN),
+  ]
   HashMap.ofList list
 
 /-- ユーザ定義の識別子なのか、言語のキーワードなのか分類する -/
-def LookupIdent (ident : String) : TokenType :=
+def LookupIdent (ident : String) : Token :=
   match keywords.find? ident with
   | some tok => tok
-  | none => IDENT
+  | none => IDENT ident