A method for manufacturing a gas electron multiplier, said GEM comprising an insulating sheet having first and second surfaces, first and second metal layers (14, 16) provided on top of said first and second surfaces, respectively, and a plurality of throughholes extending through said insulating sheet and said first and second metal layers (14, 16), said method comprising the following steps: preparing a blank sheet comprised of an insulating sheet provided with first and second metal layers (14, 16) on its first and second surfaces, respectively, said first and second metal layers (14, 16) having an initial thickness, a first metal layer hole forming step in which the first metal layer is patterned by means of photolithography, such as to form holes through said first metal layer, an insulating sheet hole forming step, in which the holes formed in the first metal layer are extended through the insulating layer by etching from the first surface side, and a second metal layer hole forming step, the method being characterized in that: the first and second metal layers (14,16) are etched from the outside, thereby reducing the initial thicknesses of the first and second metal layers (14, 16) and, simultaneously the second metal layer is etched through the holes in the first metal layer and the insulating sheet, said etching being maintained until the holes extend through the second metal layer, wherein said initial average thickness of the first and second metal layers (14, 16) is between 6.5 μm and 25 μm, preferably between 7.5 μm and 12 μm. preparing a blank sheet comprised of an insulating sheet provided with first and second metal layers (14, 16) on its first and second surfaces, respectively, said first and second metal layers (14, 16) having an initial thickness, a first metal layer hole forming step in which the first metal layer is patterned by means of photolithography, such as to form holes through said first metal layer, an insulating sheet hole forming step, in which the holes formed in the first metal layer are extended through the insulating layer by etching from the first surface side, and a second metal layer hole forming step, the method being characterized in that: the first and second metal layers (14,16) are etched from the outside, thereby reducing the initial thicknesses of the first and second metal layers (14, 16) and, simultaneously the second metal layer is etched through the holes in the first metal layer and the insulating sheet, said etching being maintained until the holes extend through the second metal layer, wherein said initial average thickness of the first and second metal layers (14, 16) is between 6.5 μm and 25 μm, preferably between 7.5 μm and 12 μm. the first and second metal layers (14,16) are etched from the outside, thereby reducing the initial thicknesses of the first and second metal layers (14, 16) and, simultaneously the second metal layer is etched through the holes in the first metal layer and the insulating sheet, said etching being maintained until the holes extend through the second metal layer, wherein said initial average thickness of the first and second metal layers (14, 16) is between 6.5 μm and 25 μm, preferably between 7.5 μm and 12 μm.