PURPOSE: To facilitate the control of the equipment along with a handier construction thereof, by inspecting light leaked at a defect with one matrix-like image sensing element shielding light over an opening to detect the defect on a conductive layer of a through hole.

CONSTITUTION: A through hole (TH) 34 is shielded with a rod 35 and is irradiated by an illumination light 42. When any defect exists in a conductive layer 33 of the TH 34, an image on the right from the center axis of the TH 34 is reflected on the surface (r) of a prism 36 and the image on the left is reflected on the surface S while the image passing straight through the TH 34 passes straight through the prism 36 to form images at parts b1, b2 and b0 of a image sensing element. The image signals are passed through a binary coding circuit 39 and the image a' formed (a) of the TH 34 unshielded is memorized into an image memory 40 and images b0'Wb2' formed at the parts b0Wb2 done into a memory 41 thereby enabling highly accurate detection of a defect of the TH 34.