This type of rotary damper generally includes a casing having an opening in one end and a bottom portion closing the other end, a rotor rotatably fitted in an end portion in the opening side of the casing, a piston movably but non-rotatably disposed in the casing, the piston being located between the rotor and the bottom portion and dividing an inner space of the casing into a first chamber in the bottom portion side and a second chamber in the rotor side and a cam member having a basal end portion disposed in the first chamber and a distal end portion rotatably and movably passing through the piston and non-rotatably connected to the rotor. 
A first cam mechanism is provided between opposite surfaces of the rotor and the piston opposed to each other.  The first cam mechanism causes the piston to be moved from the second chamber side to the first chamber side when the rotor is rotated in one direction and the first cam mechanism allows the piston to be moved from the first chamber side to the second chamber side when the rotor is rotated in the other direction.  A large-diameter portion is formed in the basal end portion of the cam member and a second cam mechanism is provided between opposite surfaces of the large-diameter portion and the piston opposed to each other.  The second cam mechanism allows the piston to be moved from the second chamber side to the first chamber side when the rotor is rotated in one direction and the second cam mechanism causes the piston to be moved from the first chamber side to the second chamber side when the rotor is rotated in the other direction.  Accordingly, regardless of direction in which the rotor is rotated, the piston is moved either by the first cam mechanism or by the second cam mechanism.  Therefore, the piston is surely moved accompanying the rotation of the rotor.