The invention of the present application relates to a manufacturing method for a bag in which two closing cords are provided at an opening part, wherein, with one end of a thermoplastic resin sheet serving as an open end and the other end serving as a closed end, after a step in which notch parts are punched out of the sheet, a tubular part for the insertion of a cord is formed, through which a first cord and a second cord, forming a pair of front and rear cords, are inserted. The first cord, which is exposed at the notch part between two adjacent bags, is fused at a prescribed location, and in the same manner the second cord is fused at a prescribed location, the cords are cut and the sheet is fusion-cut along a bag side-edge line.