PURPOSE: To enable highly economical inspection using a simple inspection jig substrate by pressing each of thick through hole land patterns of two inspection jig substrates into a clearance hole of an intermediate layer substrate in inspect an insulation state.

CONSTITUTION: With pressing bases 29 and 30, through hole land patterns 25 of a through hole 29 which is made up of a copper foil thicker than copper foils 22 of inspection jig substrates are inserted under pressure separately into holes 23 in the surface and back of an intermediate substrate 26 having the clearance holes 23 between the copper foils 22. Under such a condition, an insulation test is performed between the copper foils 22 through a changeover switch 39 and an insulation meter thereby inspecting a copper residue within the clearance hole.