The present invention is intended to elucidate a molecular target of an antidiabetic such as a thiazolidine derivative. The present invention provides a screening method for an antidiabetic, comprising the steps of: bringing a candidate substance to be screened into contact with a protein represented by the following (a) or (b): (a) a protein comprising the amino acid sequence represented by SEQ ID NO: 2; or (b) a protein comprising an amino acid sequence derived from the amino acid sequence represented by SEQ ID NO: 2 with the deletion, substitution, addition, or insertion of one or plural amino acids and interacting with the antidiabetic; and detecting the interaction between the candidate substance and the protein.