A viscous damper having improved versatility, easy to produce, capable of reducing production cost. The viscous damper comprises an annular inertial mass body (14) received in a case (10) together with a damping liquid, and a hub (11) for fixing the case on the rotary shaft of an internal combustion engine. The case comprises a case main body (12) of substantially U-shaped cross section, and a cover (7) for liquid-tightly closing an opening (12d) directed to one axial side of the main body, and defining a sealed chamber (13) in the inside. The inertial mass body is made by superposing and joining a plurality of annular plates (14a). By thrust bearings (5) attached in holes (8) in the annular plate on both axial sides of the inertial mass body and by a journal bearing (6) attached in a recessed groove (9) in the inner periphery of the inertial mass body, the inertial mass body is supported on both axial sides and radially and disposed for peripheral rotation in the sealed chamber (13).