Seat reclining apparatus

1. A seat reclining apparatus comprising:

a base plate which is fixed to one of a seat cushion side frame and a seatback side frame;

a ratchet plate which includes an internal gear, said ratchet plate being fixed to the other of said seat cushion side frame and said seatback side frame so as to face said base plate while being rotatable relative thereto;

a guide groove which is formed on a surface of said base plate which faces said ratchet plate;

a lock member guided by said guide groove so as to be movable between an engaged position, in which an outer toothed portion formed on an outer peripheral surface of said lock member is engaged with said internal gear of said ratchet plate, and a disengaged position, in which said outer toothed portion is disengaged from said internal gear of said ratchet plate toward an inner peripheral side;

a first cam which is positioned between said base plate and said ratchet plate and rotates relative to said base plate and said ratchet plate in association with an operation of operating member between a locked position to position said lock member in said engaged position and an unlocked position to position said lock member in said disengaged position; and

a second cam which moves toward said lock member and presses said lock member to said engaged position by receiving a force from said first cam when said first cam rotates from said unlocked position to said locked position,

wherein said lock member linearly moves in a radial direction, with respect to a center of rotation of said ratchet plate relative to said base plate, between said engaged position and said disengaged position,

wherein said second cam includes a surface contact portion which comes in surface contact with a common guide surface, along which said lock member also slides, of said guide groove; an engaging portion which is pressed by said first cam; and a pressing portion which presses a pressed portion of said lock member, and

wherein one of said pressing portion of said second cam and said pressed portion of said lock member includes a flat surface, and the other of said pressing portion of said second cam and said pressed portion of said lock member includes a curved surface that is convex toward said flat surface,

wherein said pressing portion of said second cam and said pressed portion of said lock member comprise said flat surface and said curved surface, respectively,

wherein said surface contact portion of said second cam comprises a flat surface.

2. The seat reclining apparatus according to claim 1 , wherein said engaging portion of said second cam comprises a curved surface that is convex toward said first cam, and

wherein the curvature of said curved surface of said engaging portion of said second cam is greater than the curvature of said curved surface of said other of said pressing portion of said second cam and said pressed portion of said lock member.

3. The seat reclining apparatus according to claim 1 , wherein said guide groove comprises a plurality of guide grooves,

wherein said lock member comprises a plurality of lock members which are guided by said plurality of guide grooves, respectively, and

wherein said second cam operates in association with one of said plurality of lock members.

4. The seat reclining apparatus according to claim 1 , wherein said second cam is symmetrical in shape with respect to a straight line passing between said surface contact portion and said pressing portion.