PURPOSE: To make it possible to perform inspections in high accuracy and high efficiency by covering the opening of a through hole (TH) with a light screening mask, and performing the two-step inspections of general inspection and precise inspection for the presence or absence of a defect in each TH based on the leaking-light inspection signal of the irradiated inspecting light.

CONSTITUTION: In a printed wiring board to be inspected 1, a TH 4 is formed. The inner surface of the TH 4 undergoes metal plating 5. A swelling land part 6 is provided at the periphery of the opening of the TH 4. The opening of the TH 4 is covered with a light screening mask 7. Inspecting light 8 is irradiated on this side. The amount of the light leaking from the TH 4 after the scanning of the board 1 is detected with a leaking light detector 30 which is arranged on the other surface side. A first detecting means 31 performs the general inspection for each TH based on the detected leaking light signal for every scanning. With respect to the specified TH for which the presence of the defect is judged as a result, a second inspecting means 32 inspects the presence or absence of the defect in detail based on the detected leaking light signals for all of the plurality of scannings. The processing of the means 31 is quick, but the excessive detections are many. The processing of the means 32 is slow, but the detecting accuracy is high. Therefore, the number of the THs to be inspected is decreased with the means 32, the processing time of the means 32 is shortened and the erroneous reporting ratio is decreased by the means 32.