A cooling system includes a cooling circuit that has a circulation pump circulating a coolant containing antifreeze solution and cools down, by means of the coolant, a power element mounted in a power converter for an on-vehicle rotary electric machine; a control signal calculation unit that calculates heat quantity generated by the power element; a power element temperature sensor that detects the temperature of the power element; and a coolant temperature sensor that detects the temperature of the coolant. The control signal calculation unit calculates, on the basis of the heat quantity, the temperature of the power element, and the coolant temperature, power element cooling performance that is the amount of heat transferred from the power element to the coolant per unit temperature difference, and lowers the driving power of the circulation pump when the calculated power element cooling performance is greater than a predetermined criterion.