PURPOSE: To completely eliminate returning light by obliquely introducing laser light from out a room to a wafer contained in a vacuum chamber, and obliquely disposing the exit window of a cylinder on the emitting side with respect to the axial center of the cylinder in case of obliquely irradiation of light reflected on the wafer to detect foreign material on the wafer.

CONSTITUTION: A wafer 2 and a work attached thereto are contained in a vacuum chamber 1, and a cylinder for introducing laser light 4 to the upper surface hole of the chamber 1 and a cylinder for radiating the reflected to laser light respectively disposed obliquely at 45° with respect opposite directions. A random reflection detector 7 is disposed opposite to the wafer 2 between the cylinders to detect dust adhered to the wafer 2. When thus constructed, the light 4 from a laser 3 is irradiated through a scanner 8 and an incident window 5 to the wafer 2, and the reflected light is irradiated from an exit window 9 provided on the cylinder 9a, the window 9 is provided to the oblique surface 9b at 45° with respect to the axial center of the cylinder 9a to eliminate the returning light into the chamber.