A damper includes a housing, a cap, a rotor portion, and blade portions projecting from the rotor portion. The blade portions include deformation portions in which the blade portions expand the diameter in one rotational direction in response to a resistance received from a viscous fluid, and in which the blade portions reduce the diameter in the other rotational direction. The blade portions engage a ring member provided on an inner circumference of the housing at a time of diameter expansion, and drag the ring member around.