It was shown that bombesin 2 (BB2) receptor antagonists typified by RC-3095 are therapeutic agents for irritable bowel syndrome (IBS), and show excellent efficacy in treating both an abdominal symptom and bowel movement disorder. Thus, the present invention provides a therapeutic agent for irritable bowel syndrome (IBS) which comprises, as an active ingredient, a bombesin 2 (BB2) receptor antagonist as well as a method for treating IBS.