diff --git a/include/picrin/irep.h b/include/picrin/irep.h
index b55b7ec0..00b53110 100644
--- a/include/picrin/irep.h
+++ b/include/picrin/irep.h
@@ -30,6 +30,7 @@ enum pic_opcode {
   OP_SUB,
   OP_MUL,
   OP_DIV,
+  OP_MINUS,
   OP_EQ,
   OP_LT,
   OP_LE,
diff --git a/src/vm.c b/src/vm.c
index cd450224..6a0392d2 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -260,7 +260,7 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value argv)
     &&L_OP_GREF, &&L_OP_GSET, &&L_OP_LREF, &&L_OP_LSET, &&L_OP_CREF, &&L_OP_CSET,
     &&L_OP_JMP, &&L_OP_JMPIF, &&L_OP_CALL, &&L_OP_TAILCALL, &&L_OP_RET, &&L_OP_LAMBDA,
     &&L_OP_CONS, &&L_OP_CAR, &&L_OP_CDR, &&L_OP_NILP,
-    &&L_OP_ADD, &&L_OP_SUB, &&L_OP_MUL, &&L_OP_DIV,
+    &&L_OP_ADD, &&L_OP_SUB, &&L_OP_MUL, &&L_OP_DIV, &&L_OP_MINUS,
     &&L_OP_EQ, &&L_OP_LT, &&L_OP_LE, &&L_OP_STOP
   };
 #endif
@@ -577,6 +577,21 @@ pic_apply(pic_state *pic, struct pic_proc *proc, pic_value argv)
       NEXT;
     }
 
+    CASE(OP_MINUS) {
+      pic_value n;
+      n = POP();
+      if (pic_int_p(n)) {
+	PUSH(pic_int_value(-pic_int(n)));
+      }
+      else if (pic_float_p(n)) {
+	PUSH(pic_float_value(-pic_float(n)));
+      }
+      else {
+	pic->errmsg = "unary - got a non-number operand";
+      }
+      NEXT;
+    }
+
 #define DEFINE_COMP_OP(opcode, op)				\
     CASE(opcode) {						\
       pic_value a, b;						\