An image forming apparatus includes an image carrier that rotates by power generated by a driving source. A power transmission system transmits the power from the driving source to the image carrier. The power transmission system includes a coupling member to attenuate oscillation. The coupling member includes a viscous fluid, male and female fittings, and a rotating resistor. The viscous fluid provides a resistance against rotation of the image carrier. The male and female fittings are mutually fitted and rotatably supported on a rotary shaft of the image carrier penetrating through the male and female fittings in a direction in which they are mutually fitted. The rotating resistor is accommodated together with the viscous fluid in an accommodation space defined by a recess of the male fitting and a bottom inner surface of the female fitting. The rotating resistor integrally rotates with the rotary shaft.