This invention provides a viscous heater which can attain an improvement in heat generating efficiency while preventing leakage caused by expansion of a viscous fluid. For this purpose, a disk-shaped rotor is employed, and a front housing body, a front plate, a rear plate, and a rear housing body constitute housings, and these are stacked and fastened together by through bolts. Surplus spaces in the shape of concaves are formed in the front plate between the respective through bolts and water passages so as to communicate with the heat generating chamber in their outer circumference.