a method and apparatus for detecting through - hole voids in a multi - layer printed circuit board . the through - hole is formed with an electrical conductor for interconnecting wiring patterns of upper layers and wiring patterns of lower layers of the printed wiring board and is illuminated with a beam having a wavelength falling within a specified wavelength band capable of exciting an luminescent beam from a layer material . the layer material is exposed by a through - hole void in the through - hole of the printed wiring board . the luminescent beam excited by the illumination beam impinging upon the layer material by way of the through - hole void in the through - hole is focussed by means of a focussing optical system and a photoelectric converter . the photoelectric converter converts the luminescent beam into an electrical signal . the presence of a through - hole void is indicated by the electrical signal .