To certainly provide a gusseted pocket part provided to a pocketbook cover or the like to a cover main body with good shapeability in high strength at a lower cost by plane processing, to facilitate the setting of a processing condition at the time of welding of a synthetic resin sheet and to form a shape enhanced in safety from an aspect of processing work.

A rectangular cover main body 1, which consists of front and back covers 11 and 12 comprising a synthetic resin sheet and a back cover 13, and a support piece 2, which comprises a synthetic resin sheet of which the back side is opened to the inner surface of at least one front cover 11 of the cover main body 1 and the other outer edge part is fixed to the outer edge of the cover main body 1, are provided and, further, a pocket part 3 comprising housing part pieces 31 and 32 capable of housing a thick article is provided to the inner surface of the back cover 12 to constitute a cover. Gusset pieces 4 bendable in a thickness direction are provided to respective sides on the peripheral side other than the sides on the opening side of the housing part pieces 31 and 32 and the end edges of the respective gusset pieces 4 held to an opposed contact state are mutually overlapped and welded to form a welded end edge 41 to constitute a bag-shaped pocket part 3 and the housing part piece 32 of the pocket part 3 is provided to the inner surface of the other back cover 12 of the cover main body 1 along the welded end edge 41 of each gusset piece 4.