Sustained-release formulation for oral administration of metformin

An object of the present invention maintains uniform over a long period of time the rate of release of metformin is to provide sustained release formulations of metformin that can be easily manufactured. Sustained-release preparation according to the invention, by using a combination of polyethylene oxide and a natural gum as a sustained release carrier, to release as metformin is active ingredient exhibits a uniform and sustained efficacy over a long period of time it can, is economical can be manufactured in a simple manner which does not require special equipment.