According to the present invention, an oil storage chamber for temporarily accumulating a high-viscosity oil is formed below a heat-generating chamber for accumulating high-viscosity oil which generates heat when a shearing force is applied thereto. When an electromagnetic coil of an electromagnetic clutch is set off, i.e., a rotation of a rotor of a viscous heater is stopped, the high-viscosity oil in the heat-generating chamber moves into the oil storage chamber by own weight thereof, and a liquid level of the high-viscosity oil in the heat-generating chamber is greatly reduced. In this way, when the electromagnetic coil of the electromagnetic clutch is turned on to start the rotor of the viscous heater, a torque applied to the rotor is greatly reduced, with the result that a stress applied to the rotor is reduced.