To facilitate work for positioning a cover in cross direction and to reduce a bookbinding adjustment time.

An apparatus is equipped with a pair of nip plates and a pair of cover mounting tables 21a, 21b which are installed on both sides of a bottom plate and above the nip plates and in which a cover positioning guide is installed on the surface on which a cover to be stuck to a main body is mounted. The positioning of the cover to be stuck to the main body is done by the positioning guide, and the positional dislocation of the cover is corrected by moving both tables 21a, 21b minutely in the same direction by the same quantity by the rotational operation of a handle 33 of a cam shaft of a minute movement adjusting mechanism in which cam shafts 30a, 30b which are installed opposite to each of the table 21a, 21b and brackets 28a, 28b which are engaged with a guide shaft 27 which is in contact with the cams of the cam shafts 30a, 30b, backs of the tables 21a, 21b, are installed.