A metallic plate member having at least one through hole to be a part a common liquid reservoir of a liquid ejection head is provided. In a first die, a plurality of first projections are arrayed in a first direction with a fixed pitch. Each of the first projections is elongated in a second direction perpendicular to the first direction. The plate member is mounted on a second die, and a regulating member is inserted into the through hole. The first projections are pressed against a first region in a first face of the plate member which is adjacent to the through hole in the second direction, so as to generate a plastic flow of a material in the plate member into gaps defined between the first projections, while the plastic flow is regulated by the regulating member. A plurality of recesses formed by the first projections and a plurality of partition walls formed by the material flown into the gaps constitutes a part of the pressure generating chambers.