A viscous fluid type heat generator adopted for being incorporated in a heating system of an automobile has a generally cylindrical hollow housing assembly in which a rotor element in the shape of an axially elongated hollow cylindrical element is rotatably received by being supported on a rotating drive shaft. The elongated rotor element has an outer circumference and end faces to cooperate with an inner wall of the housing assembly to define a heat generating chamber filled with viscous fluid, and a spiral heat receiving chamber hermetically separated from the heat generating chamber and permitting heat exchanging liquid to spirally flow therein to thereby receive heat from the heat generating chamber. The outer circumference of the elongated rotor element has a radius R and an axial length L designed so as to have such a relationship that L is larger than R.