A rotary damper comprising: a casing having an opening in one end portion thereof and a bottom portion in the other end portion thereof; a rotor (2, 2A) disposed inside the one end portion of the casing in the opening side such that the rotor is rotatable about a rotation axis; a piston (3, 3A) disposed inside a portion of the casing between the rotor and the bottom portion such that the piston is non-rotatable but movable in the direction of the rotation axis, the piston dividing an inner space of the casing into a first chamber in the bottom portion side and a second chamber in the rotor side; fluid filled in the first chamber and the second chamber; a first cam mechanism and a second cam mechanism provided between the rotor and the piston, the first cam mechanism and the second cam mechanism causing the piston to be moved in the direction of the rotation axis accompanying rotation of the rotor; wherein the first cam mechanism is formed between opposite surfaces of the rotor and the piston opposed to each other; a shaft portion extending toward the bottom portion is formed in a central portion of an end surface of the rotor opposed to the piston; characterised in that at least one outwardly projecting portion is formed in an outer peripheral surface of the shaft portion; a receiving portion in which the shaft portion is rotatably inserted is formed in a central portion of the opposite surface of the piston opposed to the rotor; at least one inwardly projecting portion is formed in an inner peripheral surface of the receiving portion; the at least one inwardly projecting portion is disposed nearer to the rotor than the at least one outwardly projecting portion; and the second cam mechanism is provided between opposite surfaces of the at least one outwardly projecting portion and the at least one inwardly projecting portion opposed to each other in the direction of the rotation axis. a casing having an opening in one end portion thereof and a bottom portion in the other end portion thereof; a rotor (2, 2A) disposed inside the one end portion of the casing in the opening side such that the rotor is rotatable about a rotation axis; a piston (3, 3A) disposed inside a portion of the casing between the rotor and the bottom portion such that the piston is non-rotatable but movable in the direction of the rotation axis, the piston dividing an inner space of the casing into a first chamber in the bottom portion side and a second chamber in the rotor side; fluid filled in the first chamber and the second chamber; a first cam mechanism and a second cam mechanism provided between the rotor and the piston, the first cam mechanism and the second cam mechanism causing the piston to be moved in the direction of the rotation axis accompanying rotation of the rotor; wherein the first cam mechanism is formed between opposite surfaces of the rotor and the piston opposed to each other; a shaft portion extending toward the bottom portion is formed in a central portion of an end surface of the rotor opposed to the piston; characterised in that at least one outwardly projecting portion is formed in an outer peripheral surface of the shaft portion; a receiving portion in which the shaft portion is rotatably inserted is formed in a central portion of the opposite surface of the piston opposed to the rotor; at least one inwardly projecting portion is formed in an inner peripheral surface of the receiving portion; the at least one inwardly projecting portion is disposed nearer to the rotor than the at least one outwardly projecting portion; and the second cam mechanism is provided between opposite surfaces of the at least one outwardly projecting portion and the at least one inwardly projecting portion opposed to each other in the direction of the rotation axis.