The present invention aims to provide a novel agent for treating and/or preventing diabetes which agent can not only control a blood sugar level but also fundamentally treat a patient with type I diabetes suffering from destruction of .beta. cells and a patient with type II diabetes suffering from dysfunction in insulin secretion. The present invention provides an agent for treating and/or preventing diabetes, the agent containing retinoic acid as an active ingredient. Retinoic acid incorporated as an active ingredient may be all-trans retinoic acid, an isomer thereof, a derivative thereof, a salt thereof or a prodrug thereof. Retinoic acid may be incorporated singly. Alternatively, composite particles of retinoic acid and an appropriate inorganic or organic substance are prepared, and the retinoic acid composite particles may be incorporated.