A lead frame is conveyed along a convey direction in a convey path filled with a reducing gas. A semiconductor pellet is placed on the lead frame at a die bonding portion. A bonding wire made of copper or a copper alloy is supplied to the next wire bonding portion. The lower end of the bonding wire is melted with an oxyhydrogen torch surrounded by an air curtain, thereby forming a ball. The bonding wire is guided into the convey path by a capillary. The ball is thermocompressed to an electrode pad of the semiconductor pellet. The other end of the bonding wire is fused and thermocompressed against the outer lead of the lead frame at a postbonding portion. The bonding wire is thus looped between the semiconductor pellet and the outer lead.