A seat reclining apparatus for a vehicle seat, the vehicle seat including a seat cushion and a seat back, the seat reclining apparatus comprising: a base member fixed to one of a lower bracket on a side of the seat cushion and an upper bracket on a side of the seat back; a cover member fixed to the other of the lower bracket and the upper bracket, the cover member being overlapped with the base member and rotatable relative to the base member; an internal tooth gear formed in an inner periphery of the cover member; three tooth plates (12A-12C) disposed between the base member and the cover member along a circumferential direction of the base member and the cover member, the three tooth plates (12A-12C) having external tooth gears which are displaceable in a radial direction of the internal tooth gear and meshable with the internal tooth gear, respectively; a cam member pivotably disposed on an inner peripheral side of the three tooth plates (12A-12C), the cam member serving to displace the three tooth plates (12A-12C) between a lock position in which the internal tooth gear and the external tooth gears are meshingly engaged with each other and an unlock position in which the internal tooth gear and the external tooth gears are disengaged from each other; a biasing member biasing each of the three tooth plates (12A-12C) in a lock direction in which the internal tooth gear and the external tooth gears come into meshing engagement; three guide projections (21a-21c) formed on the three tooth plates (12A-12C), respectively, the three guide projections (21a-21c) projecting toward the cover member; characterized by an annular grooved portion formed in a surface of the cover member which faces the base member in a concentric relation to the internal tooth gear, the annular grooved portion being recessed in an axial direction of the cover member; and three guide surfaces formed on the annular grooved portion corresponding to the respective three guide projections (21a-21c) that project into the annular grooved portion, one or two of the three guide surfaces being formed on an inner peripheral wall surface of the annular grooved portion, the remainder of the three guide surfaces being formed on the outer peripheral wall surface of the annular grooved portion, wherein the three guide surfaces serve to restrain displacement of the three tooth plates (12A-12C) toward the lock position when the respective three guide projections (21a-21c) are allowed to run upon the corresponding guide surfaces. a base member fixed to one of a lower bracket on a side of the seat cushion and an upper bracket on a side of the seat back; a cover member fixed to the other of the lower bracket and the upper bracket, the cover member being overlapped with the base member and rotatable relative to the base member; an internal tooth gear formed in an inner periphery of the cover member; three tooth plates (12A-12C) disposed between the base member and the cover member along a circumferential direction of the base member and the cover member, the three tooth plates (12A-12C) having external tooth gears which are displaceable in a radial direction of the internal tooth gear and meshable with the internal tooth gear, respectively; a cam member pivotably disposed on an inner peripheral side of the three tooth plates (12A-12C), the cam member serving to displace the three tooth plates (12A-12C) between a lock position in which the internal tooth gear and the external tooth gears are meshingly engaged with each other and an unlock position in which the internal tooth gear and the external tooth gears are disengaged from each other; a biasing member biasing each of the three tooth plates (12A-12C) in a lock direction in which the internal tooth gear and the external tooth gears come into meshing engagement; three guide projections (21a-21c) formed on the three tooth plates (12A-12C), respectively, the three guide projections (21a-21c) projecting toward the cover member; characterized by an annular grooved portion formed in a surface of the cover member which faces the base member in a concentric relation to the internal tooth gear, the annular grooved portion being recessed in an axial direction of the cover member; and three guide surfaces formed on the annular grooved portion corresponding to the respective three guide projections (21a-21c) that project into the annular grooved portion, one or two of the three guide surfaces being formed on an inner peripheral wall surface of the annular grooved portion, the remainder of the three guide surfaces being formed on the outer peripheral wall surface of the annular grooved portion, wherein the three guide surfaces serve to restrain displacement of the three tooth plates (12A-12C) toward the lock position when the respective three guide projections (21a-21c) are allowed to run upon the corresponding guide surfaces.