PURPOSE: To align an object of measurement easily, accurately, and speedily by placing the alignment area of the object of measurement and the photodetection surface of a peak value detecting means approximately in image formation relation.

CONSTITUTION: An alignment unit 20 constitutes an alignment means and a beam splitter 16 and a condenser lens 22 constitute a position detecting and separating means. Further, a CDC sensor 24 and a peak detecting circuit 26 constitute the peak value detecting means and an I/O port 42, an interface 44, a microcomputer 46, and a keyboard 48 constitute a control means. Even when the object 100 of measurement is fitted at a wring position or greatly eccentric, the alignment area and the photodetection surface of the peak value detecting means are approximately in image formation relation, so the peak position of position detection light is in the photodetection surface of the CCD sensor 24 at any time.