Disclosed are: an insulin secretion inducer an insulin secretion-inducing composition a process for the production of the composition an accelerator for increasing the number of pancreatic β-cells a composition for increasing the number of pancreatic β-cells a process for the production of the composition and a viral vector for gene therapy. The insulin secretion inducer or the accelerator for increasing the number of pancreatic β-cells comprises a polypeptide having an amino acid sequence encoded by DNA that is known to encode a membrane protein Tm4sf20 (transmembrane 4 L six family member 20) or the like or a fragment of the polypeptide as an active ingredient.
