An inspection apparatus for inspecting a wiring pattern on a printed board by illuminating the printed board with light to photoelectrically convert light reflected from the printed board into an optical grey level image corresponding to the wiring pattern thereon, the grey level image being converted into a bi-level image. The inspection apparatus includes a thinning circuit to perform a thinning process for removing the bi-level image by one picture element from the background side of the wiring pattern, the thinning process being repeatedly performed predetermined times with respect to all of picture elements of the wiring pattern to output the number of repetitions of the thinning process and further to output a skeleton image. Also included is a distance image converter to output a distance conversion image where the number of repetitions is given as a distance value from the background of the wiring pattern. The inspection apparatus compares the distance value of the distance conversion image with one or more thresholds along the skeleton image. This arrangement allows direct detection of the wiring pattern width, thereby permitting the wiring pattern inspection with a simple structure.