A method for controlling an electronically controlled thermostat provided in a cooling system of an internal combustion engine includes predicting a thermal radiation amount of a radiator when a water temperature of engine cooling water is stabilized at a second set temperature or at a first set temperature without detecting the water temperature not to cause temperature hunting, when the water temperature is controlled from a first set temperature to the second set temperature lower than the first set temperature or from the second set temperature to the first set temperature. The electronically controlled thermostat is controlled in accordance with the predicted thermal radiation amount.