ThemeTo provide a rotary damper in which a rotor and a piston can be assembled with less labor.Means to SolveA first cam mechanism 8 is provided between outer portions of opposite surfaces of a rotor 2 and a piston 3. A shaft portion 2c is integrally formed in a central portion of the opposite surface of the rotor 2 opposed to the piston 3. A pair of outwardly projecting portions 9a, 9a are formed in an outer peripheral surface of the shaft portion 2c. A through hole 3a in which the shaft portion 2c is inserted is formed in the piston 3. A pair of inwardly projecting portions 9b, 9b are formed in an inner peripheral surface of the through hole 3a. The outwardly projecting portions 9a, 9a are arranged such that each of the outwardly projecting portions 9a can pass through a gap between the inwardly projecting portions 9b, 9b and the inwardly projecting portions 9b, 9b are arranged such that each of the inwardly projecting portions 9b can pass through a gap between the outwardly projecting portions 9a, 9a. A second cam mechanism 9 is provided between opposite surfaces of the outwardly projecting portion 9a and the inwardly projecting portion 9b opposed to each other when the rotor 2 is rotated through a predetermined angle after the outwardly projecting portion 9a have passed through the gap between the inwardly projecting portions 9b, 9b.