PURPOSE: To remove the strain of a punching metal and correct the torsion of a whole filter plate by applying pressure from inside the filter plate formed in a body by welding.

CONSTITUTION: An outer stainless steel 1 is completely softened by heat treatment, and an inner stainless steel ring 2 is arranged inside the outer ring. A punching metal 3 is a hardened, this, stainless steel plate having numers of small holes, and the contact sections of the punching metal 3 with the inner and the outer rings 1, 2 are welded over the whole circumference to form a filter plate A. The filter plate A is held between a pair of upper and lower molds 4, 5 which are bound using O-rings 7, 7 at the upper and lower sides of the outer ring 1. If a fluid is introduced by pressure through a passage 8, and softened outer ring 1 is stretched in an outer circumferential direction, causing plastic deformation therein, and the punching metal 3 being perfectly stretched to a plane because of the activity of tension.