A seat reclining apparatus includes a first and second plates and a lock mechanism which locks/unlocks relative rotation therebetween. The first plate includes an engaging protrusion on a side engaged in a holding portion, and a protrusion-corresponding recess formed on the other side of the first plate recessed at a position corresponding to the engaging protrusion. The engaging protrusion includes a pressure receiving portion on a peripheral edge thereof and engages with an inner peripheral surface of the holding portion to receive a force therefrom when a load is exerted on a seatback frame in the locked state. The first plate includes an annular bridging portion connecting the peripheral edges of the engaging protrusion and the protrusion-corresponding recess; the annular bridging portion including a thin-wall portion, and a thick-wall portion corresponding to the pressure receiving portion.