A sheet stitching apparatus comprising: a frame; a clincher mounted on the frame; a stitching head arranged oppositely to the clincher for reciprocal vertical movement between a standby position in which it is upwardly separated from the clincher and a stitching position in which it abuts the clincher; a guide member mounted on the frame and guiding the stitching head in such a manner that the stitching head can be moved vertically; a drive unit moving the stitching head; a wire insertion pipe mounted on an upper end of the stitching head and extending upward from the stitching head; a wire guide consisting of a thin and long plate spring, a base end of the wire guide being fixed to an upper end of the wire insertion pipe; and a wire reel rotatably supported by the frame below a tip end of the wire guide and arranged oppositely to one side of the wire guide at its side, wherein the wire guide is bent in such a manner that its tip end is directed toward the wire reel and the one side of the wire guide is medially located, and at the same time, a wire reeled out from the wire reel is guided along the other side of the wire guide, led through the wire insertion pipe and supplied to the stitching head, and when the stitching head is in the standby position, the stitching head pulls in the wire only a predetermined length and cuts the wire, and while the stitching head lowers from the standby position, the stitching head bends the cut wire into a U-shape, and when the stitching head reaches the stitching position, the stitching head drives the U-shaped wire into a set of sheets and at the same time, the clincher bends both ends of the U-shaped wire, and a detector arranged for movement according to a change in a degree of bending of the wire guide effected by a change of a remaining amount of the wire of the wire reel and detection of a change of position of the wire guide effected by the pull-in operation of wire by the stitching head. a frame; a clincher mounted on the frame; a stitching head arranged oppositely to the clincher for reciprocal vertical movement between a standby position in which it is upwardly separated from the clincher and a stitching position in which it abuts the clincher; a guide member mounted on the frame and guiding the stitching head in such a manner that the stitching head can be moved vertically; a drive unit moving the stitching head; a wire insertion pipe mounted on an upper end of the stitching head and extending upward from the stitching head; a wire guide consisting of a thin and long plate spring, a base end of the wire guide being fixed to an upper end of the wire insertion pipe; and a wire reel rotatably supported by the frame below a tip end of the wire guide and arranged oppositely to one side of the wire guide at its side, wherein the wire guide is bent in such a manner that its tip end is directed toward the wire reel and the one side of the wire guide is medially located, and at the same time, a wire reeled out from the wire reel is guided along the other side of the wire guide, led through the wire insertion pipe and supplied to the stitching head, and when the stitching head is in the standby position, the stitching head pulls in the wire only a predetermined length and cuts the wire, and while the stitching head lowers from the standby position, the stitching head bends the cut wire into a U-shape, and when the stitching head reaches the stitching position, the stitching head drives the U-shaped wire into a set of sheets and at the same time, the clincher bends both ends of the U-shaped wire, and a detector arranged for movement according to a change in a degree of bending of the wire guide effected by a change of a remaining amount of the wire of the wire reel and detection of a change of position of the wire guide effected by the pull-in operation of wire by the stitching head.