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 KC1/HC1 buffer at 37°C as a test solution.