An object of the present invention is to provide a pharmaceutical agent which is administered to a patient with chronic kidney disease in which the progress of worsening of renal function is rapid whereby the progress of the symptom is suppressed or improved. The present invention relates to a progress-suppressing or improving agent for chronic kidney disease containing 5-hydroxy-1-methylhydantoin as an active ingredient. The pharmaceutical agent of the present invention showed a significant effect in a patient with chronic kidney disease where progress of the renal function decrease is rapid. The present pharmaceutical agent is very useful as a highly safe pharmaceutical agent which suppresses or improves the progress of worsening of the renal function of a patient with rapidly progressive chronic kidney disease for which there has been no effective therapeutic agent being simply and easily ingestible.
