A first through hole is formed in a laminate by trepanning machining using laser light, for example. The trepanning machining refers to a process for irradiating laser light along such a trajectory that the laser light is first irradiated onto a substantially central area of a first through hole to be formed, the laser light is irradiated along a circumference corresponding to the bore diameter of the first through hole to be formed, and the laser light is finally irradiated onto the substantially central area of the first through hole to be formed again. A second through hole being substantially concentric with and having a larger bore diameter than the first through hole is formed by irradiating the laser light, similarly to the first through hole.