A viscous fluid type supplementary heat generator having a housing assembly defining a fluid containing chamber including a hermetically sealed heat generating chamber in which a viscous fluid is filled to generate heat when the viscous fluid is subjected to shearing action applied by the rotation of a rotor element rotatably arranged in the heat generating chamber. The fluid containing chamber has a predetermined substantial volume with respect to which the volume of the viscous fluid filled in the fluid containing chamber is set at a value between 50% through 70%, i.e., the filling rate of the viscous fluid relative to the substantial volume of the fluid containing chamber of the viscous fluid type heat generator is determined to be a value between 0.5 through 0.7. The determination of the filling rate of the viscous fluid is made so as to protect an oil sealing device sealing the heat generating chamber from being damaged or broken by an excessive pressure prevailing in the heat generating chamber.