The present invention relates to a gastric-retention type sustained-release formulation of metformin as an insulin-independent diabetic agent, and a preparation method thereof. More specifically, the present invention relates to a gastric-retention type sustained-release formulation, which can effectively control drug release even with a small amount of a polymer by forming a sustained-release granulated material, which comprises granules containing metformin or a pharmaceutically acceptable salt thereof and a swellable polymer, and a water-insoluble coated on the surface of the granules and also can allow a tablet containing the drug to swell rapidly at an early stage by formulating via mixing of a superdisintegrant, which is capable of controlling the speed of gelation forming hydrogels, with the granulated material and a preparation method thereof
