A measurement unit (6) is provided with a fixed body (19) which is fixed on a base (17) and has a reference surface (19a). A movable body (21) slides in the base along a guide rail, and a measurement body (22) having a measuring surface (22b) slides in the movable body along the guide rail. The measurement body is always pressed against a first side wall (21a) of the movable body by a coil spring (24). A first sensor (27) to detect the time when a book body is located on the front side of the reference surface, and a second sensor (28) to detect the time when the measurement body is separated from the first side wall of the movable body by a predetermined distance, are provided.