A viscous fluid type heat generator having a heat generating chamber in which heat generation by the viscous fluid is performed in response to the rotation of a rotor element applying a shearing action to the viscous fluid, a heat receiving chamber in which a heat exchanging liquid flows to receive heat from the heat generating chamber, a heat generation control chamber for containing the viscous fluid to be supplied into the heat generating chamber and receiving the viscous fluid withdrawn from the heat generating chamber. The heat generator has a fluid supplying passage for supplying the viscous fluid from the heat generation control chamber to the heat generating chamber, a fluid withdrawing passage for withdrawing the viscous fluid from the heat generating chamber into the heat generation control chamber, a flap valve deformable to open and close the fluid withdrawing passage in response to a change in a heating requirement, and a valve deformation supplementing means to provide the flap valve with additional deformation effect to quickly and timely open the flap valve. The valve supplementing means may be a subsidiary fluid port to introduce therein the viscous fluid from the heat generating chamber so as to cause a supplementary deformation of the flap valve.