The vasopressin V2 receptor antagonist (Tolvaptan) is a new diuretic that selectively promotes the excretion of water. It has been reported to be effective for patients in cardiology, but there have been no reports of its use in the perioperative period after cardiac surgery. We report the usefulness of Tolvaptan for postoperative fluid management in a patient with severe mitral regurgitation due to ischemic cardiomyopathy. Marked improvement was achieved after administration of Tolvaptan for heart failure in the postoperative period.