A rotary damper (D) is constituted by: housings (11, 51); a silicon oil (21) being housed inside the housings (11, 51); a rotor (31) wherein a resistive portion (36) moving through the silicon oil 21 inside the housings (11, 51) is provided in an axial portion (32) which is housed inside the housings (11, 51) and projects from the housings (11, 51); and an O-ring (61) preventing the silicon oil (21) from leaking between the axial portion (32) and the housing (51), and multiple arc-like through-bores (37) are provided in the resistive portion (36) on a concentric circle, and depressed grooves (38) communicating with the arc-like through-bores (37) are provided. Accordingly, during the assembly, the air being mixed into the housing is not allowed to be excessively compressed, so that even if the rotor rotates bi-directionally, the noise being caused by the air being mixed into the housing can be prevented.