A bearing bush, disposed in a shaft hole of an external gear of a seat reclining device, which slidingly contacts wedge-shaped pieces. The seat reclining device has a differential transmission mechanism for adjusting an open angle of a hinge connecting a seating surface of a seat and a backrest thereof to each other. The differential transmission mechanism has an internal gear; an external gear, having a smaller number of teeth than the internal gear, which engages the internal gear; a pair of the wedge-shaped pieces disposed in a circular arc-shaped gap formed between the shaft hole of the external gear and a shaft of the internal gear with tapered ends of the wedge-shaped pieces opposed to each other; and a compression coil spring for imparting an elastic force to the wedge-shaped pieces in a direction in which the wedge-shaped pieces depart from each other. When the wedge-shaped pieces are moved in sliding contact with the bearing bush inside the circular arc-shaped gap with the wedge-shaped pieces being pressed by a cam provided on the shaft of the internal gear, the external gear disengages from the internal gear, whereby the open angle of the hinge is adjustable. A sliding surface of the bearing bush is formed of a fluorocarbon resin composition containing fluorocarbon resin to which thermoplastic resin, carbon fibers, and molybdenum disulfide are essentially added.