A pulley is fixed to a housing of a water pump, the pulley being connected to an output shaft of an internal combustion engine so that the pulley is driven through the output shaft. In the housing, a slider moves to and fro due to a change in pressure in a pressure chamber, which causes the magnetic flux passing through a magnet and an inductor ring of a rotary cylinder to vary, so that the driving force of the internal combustion engine is variably transmitted to the rotary cylinder. The pressure chamber is connected to an intake air passage and an atmospheric air introducing portion through a pressure path, and the state of communication is changed by a VSV. The pressure passage is provided with an orifice for preventing rapid change in pressure in the pressure chamber when the VSV is switched.