In a rotary damper 1 including a cam surface 3f formed in a lower end surface of a large-diameter portion 3b of a rotor 3 and including a cam surface 4f formed in an upper end surface of a piston 4, the cam surface 4f being capable of contacting the cam surface 3f, the cam surfaces 3f, 4f pressed to contact each other by a coil spring 9 biasing the piston 4, the piston 4 is prohibited from being moved toward the large-diameter portion 3b beyond a predetermined second position. Coil spring 9 does not rotationally bias the piston 4. The coil spring 9 biases the piston 4 only such that the piston 4 approaches the large-diameter portion 3b.