A viscous fluid type heat generator which includes a housing assembly defining a heat generating chamber (5) and a heat receiving chamber (13,14) arranged adjacent to the heat generating chamber via a partition wall (2,3) disposed therebetween. A rotor element (11) is mounted to be rotated by a drive shaft (8) for rotation within the heat generating chamber (5). A viscous fluid is held in a gap defined between the inner wall surfaces of the heat generating chamber (5) and the outer faces of the rotor element (11), for heat generation under shearing stress applied by the rotation of the rotor element (11). A heat exchanging fluid circulates through the heat receiving chamber (13,14) to receive heat transferred through the partition wall from the heat generating chamber. A plurality of grooves (23a,23b) are formed on at least one of the inner wall surfaces of the heat generating chamber (13,14) to increase a total heat transfer surface area in the inner wall surfaces, and thus to enhance a heat transfer efficiency through the partition wall. The density of arrangement of the grooves (23a,23b) in the outer peripheral area of the inner wall surface is larger than that in the inner peripheral area of the latter.