A seat reclining apparatus comprising: a lower arm adapted to be supported by a seat cushion; an upper arm adapted to be supported by a seat back and supported by the lower arm so as to be rotatable relative to the lower arm; a plurality of pawls (31A, 31B, 31C, 31D) provided at an inside of the lower arm so as to be movable in a radial direction thereof along guide walls provided at the lower arm and including external teeth (44, 47), which are engageable with and disengageable from internal teeth formed on an inner circumferential surface of the upper arm; a cam including a cam surface, at which the cam contacts a cam contacting portion (45, 48) formed on each of the plurality of pawls (31A, 31B, 31C, 31D), and pressing the pawls (31A, 31B, 31C, 31D) in a direction by which the external teeth (44, 47) of the pawls (31A, 31B, 31C, 31D) mesh with the internal teeth of the upper arm in response to a rotation of the cam in a locking direction; a biasing member arranged within an accommodating portion, which is formed on a rotational center portion of the lower arm, engaged with an inner circumferential surface of the accommodating portion at a first end portion, engaged with the cam at a second end portion and biasing the cam so as to rotate the cam in the locking direction; a restricting protrusion portion formed on a surface of the cam facing the lower arm and slidably contacting the inner circumferential surface of the accommodating portion in order to avoid an eccentricity of the cam when being rotated; characterized by an engagement protruding portion provided on the surface of the cam facing the lower arm at a position displaced from the restricting protrusion portion in a radially inward direction of the cam and engaging therewith the second end portion of the biasing member. a lower arm adapted to be supported by a seat cushion; an upper arm adapted to be supported by a seat back and supported by the lower arm so as to be rotatable relative to the lower arm; a plurality of pawls (31A, 31B, 31C, 31D) provided at an inside of the lower arm so as to be movable in a radial direction thereof along guide walls provided at the lower arm and including external teeth (44, 47), which are engageable with and disengageable from internal teeth formed on an inner circumferential surface of the upper arm; a cam including a cam surface, at which the cam contacts a cam contacting portion (45, 48) formed on each of the plurality of pawls (31A, 31B, 31C, 31D), and pressing the pawls (31A, 31B, 31C, 31D) in a direction by which the external teeth (44, 47) of the pawls (31A, 31B, 31C, 31D) mesh with the internal teeth of the upper arm in response to a rotation of the cam in a locking direction; a biasing member arranged within an accommodating portion, which is formed on a rotational center portion of the lower arm, engaged with an inner circumferential surface of the accommodating portion at a first end portion, engaged with the cam at a second end portion and biasing the cam so as to rotate the cam in the locking direction; a restricting protrusion portion formed on a surface of the cam facing the lower arm and slidably contacting the inner circumferential surface of the accommodating portion in order to avoid an eccentricity of the cam when being rotated; characterized by an engagement protruding portion provided on the surface of the cam facing the lower arm at a position displaced from the restricting protrusion portion in a radially inward direction of the cam and engaging therewith the second end portion of the biasing member.