In a sheet stitching apparatus in which a wire W reeled out from a wire reel 5 is cut only a predetermined length, the cut wire is bent into a U-shape and driven into a set of sheets S, both ends of the U-shaped wire are bent and the set of sheets are stitched, there is provided a detector 10-15 arranged for movement according to a change in a degree of bending of the wire guide 4 effected by a change in a remaining amount of the wire of the wire reel 5 for detection of a change of position of the wire guide effected by a pull-in operation of wire by the stitching head 2.