PURPOSE: To reduce the fraction defective and to make the shape accurate by locally heating the outer circumferential part of the material to change the structure into the coarse and large crystal structure, sealing the outer circumferential part and the die and executing superplastical working with the forming gas.

CONSTITUTION: A material to be worked 1 is immersed in the water, this outer circumferential part is heated with a flame 11 of a torch 10, changed to the coarse and large crystal grain structure and the superplastic characteristic is deteriorated. This treated material 1 to be worked is arranged between an upper die 2 and a lower die 3, a forming gas is supplied between the material to be worked and the upper die 2 and the superplastic working is executed. Because the coarse grain range 1a with the deteriorated superplastic characteristic is made to a sealing part, the material to be worked 1 is not bitten with the upper die 2, not broken, and leaking of the pressurized gas can be prevented. The fraction defective at the producing time can be reduced and the accurate shape can be made.