The purpose of the present invention is to provide a drug which is administered to patients suffering from chronic kidney disease that involves rapid progression of kidney dysfunction, in order to improve, or prevent the progression of, the disease state. The present invention relates to an agent for improving or preventing the progression of chronic kidney disease, the agent containing as the active ingredient 5-hydroxy-1-methyl hydantoin. This drug has exhibited a remarkable effect in patients suffering from chronic kidney disease that involves rapid progression of kidney dysfunction. This drug is extremely useful as a highly safe drug for improving, or preventing progression of, kidney dysfunction in patients suffering from rapidly progressing chronic kidney disease for which there have thus far been no effective therapeutic drugs that could be easily taken.
