Disclosed are a pharmaceutical composition, a screening method, and the like which use UDP-glucose glycoprotein glycosyl transferase 1 (UGGT1) or a gene encoding the same. UGGT1 has an extremely high proliferative activity compared to known pancreatic β-cell proliferation factors thus, UGGT1 can act as a useful therapeutic agent for diabetes without any modification and is also useful as a target substance for the development of a new therapeutic agent for diabetes.