To reduce cooling capacity required by each cooler of an internal combustion engine and a motor by providing a cooler for internal combustion engine and a cooler for motor and using the cooler on the other side additionally when cooling capacity provided by either of the coolers for internal combustion engine and motor only is insufficient.

When an engine 3 only is driven, an electric water pump 8 is stopped to stop a stream of water in a motor system cooling water passage 20 and save power consumption. When load of the engine increases abruptly (at the time of abrupt acceleration) while the engine is driven, water stream control valves 17, 18 are opened to introduce cooling water in the motor system cooling water passage 20 which has a low temperature so as to cool the engine rapidly. Moreover, when load of the engine increases steadly (long ascending), water stream control valves 13, 16 to 18 are opened, and water stream control valves 14, 15 are closed to increase cooling capacity by using a radiator 2 in the motor system cooling water passage 20. On the other hand, an electromagnetic clutch pulley 11 is turned off to stop a stream of water in an engine cooling water passage when a vehicle runs by a motor 4.