The problem of the present invention is to provide a preventive and/or therapeutic agent for diabetes and/or complications of diabetes based on the novel mode of action. The protease-inhibiting compound according to the present invention is a compound represented by the general formula (I) [wherein all the symbols have the same meanings as described in the specification], its salt or solvate, or a prodrug thereof, is useful as a preventive and/or therapeutic agent for diabetes and/or complications of diabetes.