There is provided an inspection apparatus which illuminates at least part of a sample with diffused light from a diffused light source and inspects the sample based on light reflected from the sample. An illuminating chamber is defined by a wall member. The wall member has an inner wall surface for reflecting diffused light from the diffused light source, a sample-inserting opening formed through the wall member for inserting the at least part of the sample into the illuminating chamber therethrough, and a sample-observing opening formed through the wall member for permitting light reflected from the at least part of the sample to be emitted out of the illuminating chamber. The sample is observed by the use of an object-side telecentric optical system having a lens system for collecting parallel light from the light emitted from the sample-observing opening, and an aperture stop arranged at or in the vicinity of a back focal point of the lens system.