A lock mechanism for use with a rear seat, which is provided to a lateral portion of a seat back of the rear seat, wherein said seat back is foldable forwardly and downwardly, said lock mechanism including: - a latch workable for engagement with one of a pair of front and rear leg portions (30F; 30R) of a striker under a biasing force, said latch having: * a first surface (22a1) defined in the outer edge thereof; * a lock groove defined at a point behind said first surface (22a1), said lock groove being configured to allow one of said pair of front and rear leg portions (30F; 30R) to be engaged therein in an embraced manner; * a second surface (22a2) defined in an inner edge of said lock groove; * a containing space defined at a point behind said second surface (22a2), said containing space being configured to allow the front leg portion of said pair of front and rear leg portions to be contained therein, and * a third surface (22a3) defined in an inward edge of said containing space of said latch, said third surface (22a3) serving as a stopper adapted for non-sliding contact with said front leg portion to stop the latch, - characterized in that said first and second surfaces (22a1; 22a2) of said latch are each formed as a permitting guide surface which permits the latch to move past said front leg portion in a sliding contact thereon, upon said front leg portion being contacted by each respective one of said first and second surfaces (22a1; 22a2), thereby permitting said front leg portion to pass over said lock groove of the latch, and - and in that said third surface (22a3) is arranged in said inward edge of said containing space at a point where said front leg portion of said striker is to contact the third surface (22a3), and said rear leg portion of said striker is to be brought in alignment with said lock groove upon contact of said front leg portion with the third surface (22a3). - a latch workable for engagement with one of a pair of front and rear leg portions (30F; 30R) of a striker under a biasing force, said latch having: * a first surface (22a1) defined in the outer edge thereof; * a lock groove defined at a point behind said first surface (22a1), said lock groove being configured to allow one of said pair of front and rear leg portions (30F; 30R) to be engaged therein in an embraced manner; * a second surface (22a2) defined in an inner edge of said lock groove; * a containing space defined at a point behind said second surface (22a2), said containing space being configured to allow the front leg portion of said pair of front and rear leg portions to be contained therein, and * a third surface (22a3) defined in an inward edge of said containing space of said latch, said third surface (22a3) serving as a stopper adapted for non-sliding contact with said front leg portion to stop the latch, * a first surface (22a1) defined in the outer edge thereof; * a lock groove defined at a point behind said first surface (22a1), said lock groove being configured to allow one of said pair of front and rear leg portions (30F; 30R) to be engaged therein in an embraced manner; * a second surface (22a2) defined in an inner edge of said lock groove; * a containing space defined at a point behind said second surface (22a2), said containing space being configured to allow the front leg portion of said pair of front and rear leg portions to be contained therein, and * a third surface (22a3) defined in an inward edge of said containing space of said latch, said third surface (22a3) serving as a stopper adapted for non-sliding contact with said front leg portion to stop the latch, - characterized in that said first and second surfaces (22a1; 22a2) of said latch are each formed as a permitting guide surface which permits the latch to move past said front leg portion in a sliding contact thereon, upon said front leg portion being contacted by each respective one of said first and second surfaces (22a1; 22a2), thereby permitting said front leg portion to pass over said lock groove of the latch, and - and in that said third surface (22a3) is arranged in said inward edge of said containing space at a point where said front leg portion of said striker is to contact the third surface (22a3), and said rear leg portion of said striker is to be brought in alignment with said lock groove upon contact of said front leg portion with the third surface (22a3).