An object of the present invention is to provide a therapeutic agent for diabetes mellitus or insulin resistance improving agent, or to provide foods and drinks, food and drink additives, and feeds or feed additives for treating diabetes mellitus or improving insulin diabetes. In order to achieve such an object, the present invention provides a therapeutic agent for diabetes mellitus and insulin resistance improving agent comprising hydroxyproline, a hydroxyproline derivative or a pharmaceutically acceptable salt thereof or foods and drinks, food and drink additives, feeds or feed additives for treatment of diabetes mellitus or improvement of insulin resistance comprising the same.