PURPOSE: To detect fine foreign matter by placing a spatial filter which transmits light of a pattern image provided on a wafer in either optical path, making the pattern image light quantity of the optical path where the filter is placed larger than that of the other optical path, and finding the difference between or ratio of output signals by using two photoelectric converters.

CONSTITUTION: When fine foreign matter sticking on the surface of the wafer 10 is detected, the optical system consisting of a relay lens 16 formed an objective, and a relay lens 20 is provided on the wafer 10, and a wavelength separation dichroic mirror 17 and the spatial filter 19 are arranged between the lenses 16 and 20 and also put off the optical path selectively. Further, a 1st photoelectric converting element 21 is provided on the side of the lens 20 and a photoelectric converting element 23 is provided in a lateral direction of a mirror 17 across a relay lens 22. Thus, the reflected light of a laser beam irradiates the element 21 at a low irradiation angle and the reflected light of a beam irradiates the element 23 at a high irradiation angle; and their difference or ratio is used to decide the presence of the foreign matter.