The present invention aims at providing a ligand to GPR8, its DNA, etc., and more particularly, a polypeptide capable of binding to GPR8 or its amides or esters, or salts thereof, as well as its DNA, etc. The ligand to GPR8 of the present invention is useful in developing a receptor-binding assay system with the use of a GPR8 expression system, screening candidate compounds for drugs such as preventive/therapeutic agents for obesity, appetite stimulants, prolactin production inhibitors, etc.