PURPOSE: To offer the vessel inscribed on the face, which is united with costal ribs projecting toward the interior of the vessel, by protruding a film-cooling rib at the top end of a plug to be engaged with the part of a film material expected to be formed into a vessel, and pressing the film just before vessel-forming operation.

CONSTITUTION: A thermally-softened bandlike resin film F is intermittently carried and stopped at a forming position. At this time, the film F is clamped between an upper die 1 and a female die 2. A plug 3 is lowered by the operation of a cylinder 4, to press and elongate the film F with the ribbed part 31 at the top end of the plug 3. The plug 3 is kept at a fixed temperature, so as to cure a part touching the ribbing part 31 only. Consequently the elongation of the cured part is not promoted, even when the plug 3 is further lowered. Thereafter, pressurized air is introduced into a forming chamber 11, to press the film F in alignment with the forming chamber 21 of the female die 2. Thus, a vessel-forming operation is completed. The part, which came in contact with the ribbed part 31 of the plug 3, remains as a rib (a) in the formed vessel because of its different elongation rate. Thereby, the toughness of the vessel is assured.