Provided is a sustained-release preparation containing pioglitazone or a salt thereof as an active ingredient and showing superior sustainability. A sustained-release preparation containing pioglitazone or a salt thereof, which shows a dissolution ratio of pioglitazone of average 25-58% at the 2-hour time point, and average 60-100% at the 4-hour time point, in a dissolution test according to the 50 rpm USP Paddle Method and using pH 2.0 KCl/HCl buffer at 37° C. as a test solution.