A viscous fluid heat generator includes: a shear chamber in which viscous fluid is contained and heat is generated by shearing the viscous fluid with a rotor rotating therein; and a space receiving viscous fluid drained from the shear chamber. The heat generator is rotated always as far as an engine rotates because it is driven by the engine without a clutch. The viscous fluid in the shear chamber is drained into the receiving space disposed underneath the shear chamber when the rotor halts or the heat generator is not in operation. The drained viscous fluid is pushed up again into the shear chamber when the rotor is re-started or the heat generator becomes in operation again. The heat generator is re-started easily with a very small starting torque because there is almost no viscous fluid left in the shear chamber and accordingly power to shear the viscous fluid with the rotor is not required. Only a minimal power is consumed by the heat generator when heat is not required to be generated even though the rotor is continuously rotated because the viscous fluid in the shear chamber is drained in this situation.