Relaxin-3 is found to have feeding-stimulating activity, body weight increasing activity, and fat weight increasing activity when intracerebroventricularly administered to rats through observation of amount of feeding, body weight and fat weight after administration of relaxin-3. This invention includes: a polypeptide having useful effects in stimulating feeding, increasing body weight, and fattening; a therapeutic agent containing the polypeptide; a method of screening for a compound, a substance, or a salt thereof which activates or suppresses a receptor of the polypeptide; a kit for screening; and an agent with a substance which inhibits expression of the polypeptide, such as a feeding-suppressing agent, a therapeutic agent for the treatment of obesity, and a therapeutic agent for the treatment of diabetes.