Disclosed is an aperture variable inspection optical system including a variable aperture unit 13 having a polygonal light transparent section and light collecting systems 12a, 12b for forming an irradiation spot U of light passing through the variable aperture unit 13 at the position of a sample S. The variable aperture unit 13 is capable of changing the shape/size of the polygon. The size of the irradiation spot U can be changed without rearranging the aperture unit.