PURPOSE: To prevent that the flatness of a wafer in which a foreign body exists on the rear is worsened when the wafer is mounted on a wafer holder.

CONSTITUTION: The rear of a wafer 10 is scanned in the X-direction by a laser beam LBI by using a scanner 16, the wafer 10 is moved by a drive part 11, and the rear of the wafer 10 is scanned in the Y-direction by the laser beam. Scattered light from the rear of the wafer 10 is received by a photoelectric conversion element 23, and the position of a foreign body on the rear of the wafer 10 is detected. The wafer 10 is mounted on a wafer holder 2 in such a way that the foreign body on the rear of the wafer 10 is not overlapped with pairs of protrusion parts 3 to 6 on the wafer holder 2.