A gas-permeable porous fluororesin film (4) formed from a fluororesin is welded to a resin component (2) formed from a thermoplastic resin by means of a welding horn (62) that has protruding sections (62t) on a work surface (62p) that comes in contact with an object to be processed. The work surface (62p) of the welding horn (62) has a plurality of protruding sections (62t) provided thereupon. The plurality of protruding sections (62t) may be arrayed in a grid on the work surface (62p).