PROBLEM TO BE SOLVED: To provide a ligand for GPR8, and further to provide its DNA or the like. SOLUTION: A polypeptide, or a polyamide, a polyester or a salt thereof, having bondability to the GPR8 are provided. The ligand for the GPR8 is useful for the development of a receptor-binding assay system, the screening of a medicine candidate compound such as a prophylactic or therapeutic agent of obesity, an appetizer and a prolactin production-inhibiting agent, etc.