A rotary damper (1) in which a cam surface (3f) is formed on the lower end surface of a large-diameter section (3b) of a rotor (3), a cam surface (4f) making contact with the cam surface (3f) is formed on the upper end surface of a piston (4), and the cam surfaces (3f, 4f) are made to be in pressing contact with each other by a coiled spring (9) for urging the piston (4). Until the piston (4) reaches a position immediately before a second position, a pivoting restricting section (4c) is in contact with a flat section (2c) and the piston (4) is held not rotatable. Then, when the piston (4) reaches the second position, the position of the lower end of the pivoting restricting section (4c) substantially agrees with or is slightly above the position of the upper end of the flat section (2c) and the entire pivoting restricting section (4c) is located above the flat section (2c). Thus, the piston (4) can not pivot from a slope section (4b) in the direction to that portion of the flat section (4a) which excludes the pivoting restricting section (4c) but can pivot in the opposite direction until the slope section (4b) engages the flat section (2c).