A sustained-release preparation comprising - pioglitazone, or a salt thereof, - a water-soluble hydrophilic base or water-insoluble hydrophilic base as gel forming promoter and - a gel forming polymer wherein the gel forming polymer is polyethylene oxide and wherein the polyethylene oxide is 1) contained such that the product of the weight average molecular weight x 1/10000 and the content (%) of polyethylene oxide in the preparation is from (7×M+1500) to (14.5×M+5410), wherein M shows a value calculated by a weight average molecular weight of polyethylene oxide x 1/10000 when one kind of polyethylene oxide is used; or 2) contained such that the product of the weight average molecular weight x 1/10000 and the content (%) of polyethylene oxide having the highest average molecular weight in the preparation is from (14×M-300) to (17×M+7700), wherein M shows a value calculated by a weight average molecular weight of polyethylene oxide having the highest average molecular weight among the polyethylene oxides used x 1/10000 when two or more kinds of polyethylene oxides having different average molecular weights are used. - pioglitazone, or a salt thereof, - a water-soluble hydrophilic base or water-insoluble hydrophilic base as gel forming promoter and - a gel forming polymer wherein the gel forming polymer is polyethylene oxide and wherein the polyethylene oxide is 1) contained such that the product of the weight average molecular weight x 1/10000 and the content (%) of polyethylene oxide in the preparation is from (7×M+1500) to (14.5×M+5410), wherein M shows a value calculated by a weight average molecular weight of polyethylene oxide x 1/10000 when one kind of polyethylene oxide is used; or 2) contained such that the product of the weight average molecular weight x 1/10000 and the content (%) of polyethylene oxide having the highest average molecular weight in the preparation is from (14×M-300) to (17×M+7700), wherein M shows a value calculated by a weight average molecular weight of polyethylene oxide having the highest average molecular weight among the polyethylene oxides used x 1/10000 when two or more kinds of polyethylene oxides having different average molecular weights are used. wherein the gel forming polymer is polyethylene oxide and wherein the polyethylene oxide is 1) contained such that the product of the weight average molecular weight x 1/10000 and the content (%) of polyethylene oxide in the preparation is from (7×M+1500) to (14.5×M+5410), wherein M shows a value calculated by a weight average molecular weight of polyethylene oxide x 1/10000 when one kind of polyethylene oxide is used; or 2) contained such that the product of the weight average molecular weight x 1/10000 and the content (%) of polyethylene oxide having the highest average molecular weight in the preparation is from (14×M-300) to (17×M+7700), wherein M shows a value calculated by a weight average molecular weight of polyethylene oxide having the highest average molecular weight among the polyethylene oxides used x 1/10000 when two or more kinds of polyethylene oxides having different average molecular weights are used. 1) contained such that the product of the weight average molecular weight x 1/10000 and the content (%) of polyethylene oxide in the preparation is from (7×M+1500) to (14.5×M+5410), wherein M shows a value calculated by a weight average molecular weight of polyethylene oxide x 1/10000 when one kind of polyethylene oxide is used; or 2) contained such that the product of the weight average molecular weight x 1/10000 and the content (%) of polyethylene oxide having the highest average molecular weight in the preparation is from (14×M-300) to (17×M+7700), wherein M shows a value calculated by a weight average molecular weight of polyethylene oxide having the highest average molecular weight among the polyethylene oxides used x 1/10000 when two or more kinds of polyethylene oxides having different average molecular weights are used.