A heating system incorporating a heat generator confining therein a heat-generative fluid to viscously generate heat when a shearing action is applied to the fluid by a rotor element, and a heat-generation controller including a heat-generation adjusting actuator which adjustably changes the heat-generating performance of the heat generator on the basis of a signal detected as a first control signal indicating a change in the rotating speed of the rotor element and a preset reference signal. A second control signal detected to indicate a temperature of the heat-generative fluid is used to adjustably change the preset reference signal. The operation of the heating system is controlled by a method in which the first control signal is compared with the preset reference signal to determine whether or not the heat-generation adjusting actuator should actuated to change the heat-generating performance of the heat generator. The method is performed so as to adjust the preset reference signal on the basis of the second control signal which is detected by a temperature sensor.