In wire bonding in assembling of a semiconductor device, an Al wire is coupled to a lead section by a wedge which is a bonding tool, thereafter, the wedge is withdrawn from the top of the lead section and a cutter is lowered and the Al wire is cut off in this state. Lowering of the cutter is stopped at a point in time that a stopper which is lowered simultaneously with lowering of the cutter has truck against the lead section and cutting of the Al wire is terminated by stopping of lowering of the cutter.