The present invention provides a new anti-tumor agent containing adiponectin as an active ingredient, particularly, an anti-tumor agent capable of inhibiting carcinogenesis in the liver, use of adiponectin as an anti-tumor agent, and a prophylactic or therapeutic method against a tumor using adiponectin. The administration form may be either oral administration, e.g., by tablets or parenteral administration, e.g., by injection; in the case of intravenous injection for inhibiting carcinogenesis in the liver, the dosage is 1 to 100 mg/kg/day per adult patient.