An improved viscous heater is disclosed. The viscous heater has a heating chamber accommodating viscous fluid and a rotor. The rotor rotates to shear and heat the viscous fluid. The heating chamber has a pair of inner flat surfaces and an inner peripheral surface. The rotor has a pair of outer flat surfaces that respectively opposes the inner flat surfaces by predetermined gaps and an outer peripheral surface that opposes the inner peripheral surface. The outer peripheral surface has peripheral edge portions each angularly and continuously extending from the outer flat surface. The peripheral edge is chamfered so that the rotor is prevented from interfering with the inner flat surfaces.
