A variable capacity type viscous heater is provided which can carry out the capacity control reliably, and which can inhibit the endurable heat-generating efficiency of a viscous fluid from deteriorating even after a long period of service. For instance, its rear housing 6 is provided with a control chamber 9 which is communicated with a central region of a heat-generating chamber 7, and which has an internal volume capable of expanding and contracting. When a rotor 14 is kept rotated, a silicone oil held, in the heat-generating chamber 7, expands the internal volume of the control chamber 9 by the Weissenberg effect in the capacity reduction. As a result, the heating is relieved, because the silicone oil, held in the heat-generating chamber 7, is collected into the control chamber 9.