PURPOSE: To miniaturize the titled apparatus by shortening the focal distance of condensed light, by providing a tungsten electric lamp arranged to one side of an object to be detected while providing a pair of convex lenses, which are parallelly arranged so as to align the optical axes thereof, to the other side of said object to be detected.

CONSTITUTION: A tungsten electric lamp 6 being a light source is arranged above a strip article 3 to be detected and a pair of convex lenses 7, 8 are parallelly arranged thereunder so as to align the optical axes thereof. In this apparatus, light emitted from the tungsten electric lamps 6 being the light source pieces the strip article 3 if a pinhole is present and converged by convex lenses 7, 8 but a focal distance is shortened by using a pair of the lenses. By this constitution, the apparatus becomes inexpensive because a general-purpose tungsten electric lamp and a convex lens can be used and the miniaturization of the apparatus can be realized because the focal distance of converged bundle of rays is shortened.