Abstract: A collector 12 which serves as a gas diffusion layer for a fuel cell; i.e., a metal lath MR, is formed by a method consisting of a first step and a second step. In the first step, while an upper blade UH is positioned at a first machining position along the width direction of a stainless steel sheet S, generally hexagonal through holes are formed in the stainless steel sheet S at two positions which are biased from each other by a machining pitch. In the second step, while the upper blade UH is positioned at a second machining position along the width direction of the stainless steel sheet S, generally hexagonal through holes are formed in the stainless steel sheet S at two positions which are biased from each other by the machining pitch. The first step and the second step are alternated repeatedly, thereby forming the metal lath MR having a uniform shape and a predetermined thickness.