diff --git a/parse.c b/parse.c
index beaee1a2e9c307b1b4deb88f700b0219d425ebf3..9f8ae31309a1002890637ecccf7463e2732ef218 100644
--- a/parse.c
+++ b/parse.c
@@ -501,7 +501,7 @@ static Node *funcall(Token **rest, Token *tok) {
   return node;
 }
 
-// primary = "(" expr ")" | ident func-args? | num
+// primary = "(" expr ")" | "sizeof" unary | ident func-args? | num
 static Node *primary(Token **rest, Token *tok) {
   if (equal(tok, "(")) {
     Node *node = expr(&tok, tok->next);
@@ -509,6 +509,12 @@ static Node *primary(Token **rest, Token *tok) {
     return node;
   }
 
+  if (equal(tok, "sizeof")) {
+    Node *node = unary(rest, tok->next);
+    add_type(node);
+    return new_num(node->ty->size, tok);
+  }
+
   if (tok->kind == TK_IDENT) {
     // Function call
     if (equal(tok->next, "("))
diff --git a/test.sh b/test.sh
index 6e50a022da627a07e9b7912e94aed4cccc5ddfc5..c413a7753ecd6dd6b551c64a73e85242d86e1574 100755
--- a/test.sh
+++ b/test.sh
@@ -137,4 +137,17 @@ assert 3 'int main() { int x[2][3]; int *y=x; y[3]=3; return x[1][0]; }'
 assert 4 'int main() { int x[2][3]; int *y=x; y[4]=4; return x[1][1]; }'
 assert 5 'int main() { int x[2][3]; int *y=x; y[5]=5; return x[1][2]; }'
 
+assert 8 'int main() { int x; return sizeof(x); }'
+assert 8 'int main() { int x; return sizeof x; }'
+assert 8 'int main() { int *x; return sizeof(x); }'
+assert 32 'int main() { int x[4]; return sizeof(x); }'
+assert 96 'int main() { int x[3][4]; return sizeof(x); }'
+assert 32 'int main() { int x[3][4]; return sizeof(*x); }'
+assert 8 'int main() { int x[3][4]; return sizeof(**x); }'
+assert 9 'int main() { int x[3][4]; return sizeof(**x) + 1; }'
+assert 9 'int main() { int x[3][4]; return sizeof **x + 1; }'
+assert 8 'int main() { int x[3][4]; return sizeof(**x + 1); }'
+assert 8 'int main() { int x=1; return sizeof(x=2); }'
+assert 1 'int main() { int x=1; sizeof(x=2); return x; }'
+
 echo OK
diff --git a/tokenize.c b/tokenize.c
index f91c213a1ad62cfc8098d95b3e77fdb63abe1c38..e02f01a155ab1a3f8dc357c6634e79f0e8f0b267 100644
--- a/tokenize.c
+++ b/tokenize.c
@@ -82,7 +82,9 @@ static bool is_ident2(char c) {
 }
 
 static bool is_keyword(Token *tok) {
-  static char *kw[] = {"return", "if", "else", "for", "while", "int"};
+  static char *kw[] = {
+    "return", "if", "else", "for", "while", "int", "sizeof",
+  };
 
   for (int i = 0; i < sizeof(kw) / sizeof(*kw); i++)
     if (equal(tok, kw[i]))