PURPOSE: To obtain a curved plate with surely formed corrugation by pressing a punch to a die so that a plate having corrugation in the direction of an axis is formed by stretching, and allowing an auxiliary punch to make side displacement, and thereby avoiding generation of folding by forming.

CONSTITUTION: In a plate 15 used for joining part of membrane bottom and side plates of an LNG tank etc., corrugation 16, 16... are formed beforehand in two rows and in the direction of one axis and the plate is eventually formed to a curved plate 15' that has ended R and R. For this purpose, the plate 15 is set by clamping devices 29, 29. Main and auxiliary punches 31, 37, 37 are lowered to bring into contact with the plate 15. The punch 31 further lowers and pressed the plate 15 between the die 23 through folding 16. A pair of punches 37 are joined to a pair of tilting device 29. Couple of forces are generated between the punch 31 to give tensile force and bending work is effected. During while, a pair of punches 37 make sliding displacement along the circumference of the punch 31. The fold 16 is held by waveforms 39, 40, 26 of punches 31, 37 and dies 23, 24 and always in contact with the plate 15 without deformation even when the punch 37 changes position. Thus, folding by forming can be prevented.