A seat reclining mechanism comprises a guide bracket, and an internal gear member coupled to the guide bracket in a rotatable manner about a rotation axis thereof with respect to the guide bracket. A forwardmost-inclination stopper protrusion and a rearwardmost-inclination stopper protrusion are provided on either one of the guide bracket and the internal gear member. A stopper plate is fixed to a remaining one of the guide bracket and the internal gear member. The stopper plate has a forwardmost-inclination stopper portion adapted to be brought into contact with the forwardmost-inclination stopper protrusion when a seat back is at a forwardmost inclined position, and a rearwardmost-inclination stopper portion adapted to be brought into contact with the rearwardmost-inclination stopper protrusion when the seat back is at a rearwardmost inclined position.