Disclosed is a seat reclining mechanism, which is designed to be mounted to a seat cushion and a seat back so as to allow the seat back to be reclined. The 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 and formed with internal teeth along a circle having a center on the rotation axis. A plurality of lock gear members is disposed between the guide bracket and the internal gear member. A fitting portion is provided in either one of the guide bracket and the internal gear member. A rotatable cam is rotatably supported by the fitting portion.