Sustained release compositions containing a physiologically active substance or its salt, hydroxynaphthoic acid or its salt and a lactic acid-glycolic polymer or its salt, wherein the product of the weight-average molecular weight of the lactic acid-glycolic acid polymer by the amount (.mu.mol) of the terminal carboxyl group per unit mass (g) of the lactic acid-glycolic acid polymer is from 1,200,000 to 3,000,000 (inclusive); and their production; medicaments containing these sustained release compositions, etc.