The present invention relates to an agent for the prophylaxis and treatment of diabetic complications comprising, as an active ingredient, 4-[α-hydroxy-2-methyl-5-(1-imidazolyl)benzyl]-3,5-dimethylbenzoic acid, an optically active compound thereof or a pharmaceutically acceptable salt thereof. The present invention further relates to a method for the prophylaxis and treatment of diabetic complications comprising administering an effective amount of this compound. The medicament of the present invention is useful for the prophylaxis and treatment of diabetic complications, namely, diabetic neuropathy, nephropathy, ophthalmopathy, arteriosclerosis and the like. The action of the drug is long-lasting for very small doses and a single administration a day is sufficient.