Cooling system for internal combustion engine

A cooling system (20) for an internal combustion engine (11) includes a radiator (22) provided in a coolant circulation path of the internal combustion engine, and an actuator (24) that controls a flow rate of a coolant that passes through the radiator. In the cooling system, the actuator is controlled so that a coolant temperature of the engine becomes substantially equal to a target coolant temperature. The cooling system calculates a cooling loss as a quantity of heat removed from the engine by the coolant, based on an operating state of the engine, and calculates a required radiator flow rate, based on the cooling loss, the target coolant temperature, and a temperature of the coolant that has passed through the radiator. The required radiator flow rate represents a quantity of the coolant required to pass through the radiator so as to achieve the target coolant temperature. The cooling system then controls the actuator based on the required radiator flow rate.