PURPOSE: To reduce spring-back of a mask spherical peripheral part in warm press forming of an invar mask, and to improve mask deformation yield strength after forming is completed.

CONSTITUTION: This die assembly is constituted so that an air blowout port 8 is provided in a peripheral part of a punch 1 of a warm press die, warm air or air of a room temperature is fed therefrom, and a temperature of a mask stock plate 6 and a forming mask can be controlled. At the time of overhanging the mask spherical surface, a temperature of the mask stock plate 6 is raised enough before the die and the mask stock plate 6 come into contact with each other. Also, in a process in which the forming mask is allowed to ascend and taken out from a bottom dead center at the time of forming the mask, it is taken out by blowing out air of a room temperature from the air blowout port 8 and after forced cooling. In such a way, recessed deformation caused by spring-back in the periphery of the spherical part peculiar to the invar mask can be suppressed, and also, wrinkle deformation generated at the time of taking out the forming mask from the die can be prevented.