Rotary damper and assist grip device

A rotary damper (D) includes a housing (21) having an annular chamber (25); a rotor (28) with a main body (29) inserted into the annular chamber and a cap portion (30) to close the annular chamber; sealing members (32, 33) for sealing between the rotor and the housing; and a viscous fluid (35) filled in a fill space formed by the housing, rotor and sealing members. The rotor (28) is attached to the housing (21) such that an engaging shoulder portion (27, 23d) is provided on one of the housing and the rotor, and an engaging portion (31a, 30d, 23e) is provided on the other Lhereof. As the engaging portion engages the engaging shoulder portion, the rotor can be securely attached to the housing with tight sealing, and the rotor and the housing can be easily assembled.