A folding seat, comprising: a seat frame; a seat back frame cross-arranged in back of the seat frame, the seat back frame having a first side rotatably coupled to the seat frame; and a locking portion coupled at a first side thereof to the seat back frame and rotatably coupled at a second side thereof to the seat frame at a position between the seat frame and the seat back frame, wherein the locking portion is configured to be inserted into a rear portion of the seat frame or to be separated from the seat frame according to an operation thereof, wherein the locking portion comprises: a first bracket coupled to a side surface of the seat back frame; a stopper slidably coupled to the first bracket so that the first bracket is inserted into or separated from the seat frame; and a leg frame rotatably coupled to the seat frame and connected to the stopper by a cable, wherein the seat frame includes a first frame, a second frame, and an extending frame, wherein at an end portion of the second end of the second frame, a portion at a forward direction side of the seat expands and extends in a plate shape that surrounds the end portion of the second end of the second frame at the forward direction side of the seat, so that a first rotation member is formed, wherein insertion holes (222, 223) that penetrate opposite side surfaces of the first rotation member are formed at an upper portion and a lower portion toward the forward direction of the seat of the first rotation member, respectively, wherein in a state in which the seat frame is folded toward the rearward direction of the seat and the upward direction so that the seat frame is fixed toward a forward direction of the seat back frame, when the leg frame is separated from the floor of the seat frame by moving the center portion of the leg frame toward the forward direction of the seat, the stopper is separated from the insertion hole, and the seat frame is converted to a state of movable toward the forward direction of the seat and the downward direction. a seat frame; a seat back frame cross-arranged in back of the seat frame, the seat back frame having a first side rotatably coupled to the seat frame; and a locking portion coupled at a first side thereof to the seat back frame and rotatably coupled at a second side thereof to the seat frame at a position between the seat frame and the seat back frame, wherein the locking portion is configured to be inserted into a rear portion of the seat frame or to be separated from the seat frame according to an operation thereof, wherein the locking portion comprises: a first bracket coupled to a side surface of the seat back frame; a stopper slidably coupled to the first bracket so that the first bracket is inserted into or separated from the seat frame; and a leg frame rotatably coupled to the seat frame and connected to the stopper by a cable, wherein the seat frame includes a first frame, a second frame, and an extending frame, wherein at an end portion of the second end of the second frame, a portion at a forward direction side of the seat expands and extends in a plate shape that surrounds the end portion of the second end of the second frame at the forward direction side of the seat, so that a first rotation member is formed, wherein insertion holes (222, 223) that penetrate opposite side surfaces of the first rotation member are formed at an upper portion and a lower portion toward the forward direction of the seat of the first rotation member, respectively, wherein in a state in which the seat frame is folded toward the rearward direction of the seat and the upward direction so that the seat frame is fixed toward a forward direction of the seat back frame, when the leg frame is separated from the floor of the seat frame by moving the center portion of the leg frame toward the forward direction of the seat, the stopper is separated from the insertion hole, and the seat frame is converted to a state of movable toward the forward direction of the seat and the downward direction. a first bracket coupled to a side surface of the seat back frame; a stopper slidably coupled to the first bracket so that the first bracket is inserted into or separated from the seat frame; and a leg frame rotatably coupled to the seat frame and connected to the stopper by a cable, wherein the seat frame includes a first frame, a second frame, and an extending frame, wherein at an end portion of the second end of the second frame, a portion at a forward direction side of the seat expands and extends in a plate shape that surrounds the end portion of the second end of the second frame at the forward direction side of the seat, so that a first rotation member is formed, wherein insertion holes (222, 223) that penetrate opposite side surfaces of the first rotation member are formed at an upper portion and a lower portion toward the forward direction of the seat of the first rotation member, respectively, wherein in a state in which the seat frame is folded toward the rearward direction of the seat and the upward direction so that the seat frame is fixed toward a forward direction of the seat back frame, when the leg frame is separated from the floor of the seat frame by moving the center portion of the leg frame toward the forward direction of the seat, the stopper is separated from the insertion hole, and the seat frame is converted to a state of movable toward the forward direction of the seat and the downward direction.