To provide a mounting structure for a wall panel and a method for improving planarity capable of improving planarity of an upper end part of the wall panel easily and correcting a deviation of the upper end part of the wall panel easily when the upper part of the wall panel is to be attached by being protruded upward in a veranda part and a parapet part.

A horizontal member 11 which brings its collar part 11b into contact with an inner side face of a wall panel 6 and has a substantially L- shaped cross section is provided across adjacent wall panels 6 in upper end parts of them. On the other hand, barlike members 12 having a platelike terminal part 12a in an upper end part are driven into upper header faces 6a close to longitudinal joints of mutual wall panels 6, respectively, and the platelike terminal part 12a of the barlike member 12 is welding and fixed to a substantially plan part 11a of an angle 11.