To provide a flow of a coolant favorable for low speed engine idling rotation while avoiding pump cavitation during engine high speed rotation without using an auxiliary water pump.

A viscous coupling 50 is connected or coupled to a water pump 52 and it is used for controlling a flow rate of the coolant in a cooling device. When an engine is in a low speed rotation state, the water pump 52 is driven at a speed very close to an input rotational speed, and the viscous coupling hardly affects a rotational speed of the pump. But, by the existence of the viscous coupling, a large water pump can be used and the coolant can be sufficiently caused to flow even during low speed operation of the engine. In proportion to rise of a rotational speed of the engine, the viscous coupling slips and a relative input rotational speed to the water pump declines, and by this, danger of the water pump causing cavitation is reduced.