A method for estimating the amount of heat received by refrigerant by an ECU comprises a step for detecting an estimation factor including the intake air amount of an internal combustion engine, and a step for estimating the amount of heat received from exhaust gas by cooling water of the internal combustion engine in a water-cooled exhaust manifold based on a detected estimation factor. Preferably, the estimation factor further includes at least any one of the cooling water temperature, the intake air temperature of the internal combustion engine, or the number of revolutions of the internal combustion engine.