a thermostat 110 a radiator 120 a coolant temperature sensor 130 an electric water pump 140 and a control portion 150 the thermostat 110 determines a circulation direction of coolant according to a coolant temperature exhausted from the engine 100 to the bypass line or the radiator 120 such that the engine 100 is cooled by the coolant the radiator 120 expands contact area of the coolant with the air through core to rapidly emit the heat absorbed in the coolant the cooling fan 121 is disposed at one side of the radiator 120 to blow air through the radiator 120 wherein the cooling fan 121 is operated in a slow or a high speed by a control signal transferred from the control portion 150 according to driving condition and coolant temperature in a condition that the coolant temperature is higher than a predetermined value the coolant temperature sensor 130 detects a temperature of the coolant circulating the engine 100 to transmit the detected signal to the control portion 150 the coolant temperature sensor 130 can be disposed on the thermostat 110 the electric water pump 140 is disposed between the engine and the thermostat 110 to be turned on off or an operating speed thereof is controlled according to control signals transmitted from the control portion 150 to circulate the coolant the electric water pump 140 can be one of a clutch type of water pump and an electric water pump if it is determined that the engine 100 is being operated in a high speed high load condition that heat load.