To greatly improve a degree of freedom of a shape in a formable range with pressing by pressure-supplying a secondary pressure air higher than a primary pressure air from an air pressure inlet while a space maintains air tightness with an elastic body after the primary pressure is applied to the space.

A space is arranged between an upper die 1 and a lower die 2 and the space 7 of a bulged tube height is formed to a bulged tube face side of a panel material side 3. An O ring 4 presses an outer peripheral edge part of the bulged tube side of the panel material 3 by its elastic force. A primary air normally having about 30 kg/cm2 is filled from a primary air pressure inlet 5 into the space 7 encircled with the O ring. The primary air is sealed with the O ring. Successively, a secondary air normally having a pressure of about 60 kg/cm2 higher than the primary air pressure is pressure supplied by opening a secondary air 6 inlet arranged in a pattern part of a non-pressure welded part of the panel material 3 protruding at one side center and more outward than the upper/lower dies, the upper die is removed.