PURPOSE: To execute easily working of a titled diaphragm, and also to obtain a uniform product, by using a forming die which is sufficiently larger than thickness of a plastic film formed by a clearance between plugs gnawed to each other in upper and lower forming dies.

CONSTITUTION: Each clearance of a plate-like plug 11a of a upper die 11 and a plate-like plug 11'a of a lower die 11' is set sufficiently larger than thickness of a plastic film 2. When the heated plastic film 2 is placed between the die 11 and the die 11' and is pressed by use of said forming die, a core of the plastic film is obtained. According to such a forming method, first the tip of each plate- like plug of the upper die and the lower die contacts with the heated plastic film and takes away its heat, therefore, the this part is cooled and cured quickly, subsequently an uncooled plastic film between each plate-like plug is stretched and formed, therefore, uneven thickness is not generated.