A table (31) is located in a first position and a second position which is rotated at 90 degrees from the first position. A cover (27) is arranged on a top face of the table (31) and fixed to the table (31) by fixing means (32a, 32b). Cardboards are preliminarily attached to predetermined positions on a rear face of the cover (27), and an outer margin of a predetermined width is formed outside the cardboard. Folding units (35) are arranged on either side of the table (31), and guided by guide rails (34) for reciprocal linear movement in directions toward and away from the table (31). The folding units (35) are located in an outer margin folding position at which said folding units (35) engage with the cover (27) on the table (31) and fold the outer margin of the cover (27) so as to attach the outer margin to the cardboards, and a standby position at which the folding units (35) separate from the cover (27). A control unit controls the table (31) and the folding units (35). The folding units (35) are synchronized with each other in such a manner that the folding units (35) simultaneously fold the outer margin of the cover (27) each time the table (31) is located in the first and second positions.