PURPOSE: To detect the deviation of an attaching position by giving a difference in intensity between first light transmitted through a filter having apart different in light transmittance and second light transmitted through the other part of the lifter.

CONSTITUTION: Light in the center part of reflected light 39 passes pinholes 33a and 34a to become first light 39-1, and light in the other part is absorbed by 50% by each of ND filters 33 and 34 to become second light 39-2. Since the intensity of the first light 39-1 is four times as high as that of the second light 39-2, a difference of brightness appears between images 40-1 and 40-2 as shown by a line 43 and the image 40-1 is bright but the image 40-2 is dark. When a prism 1 is bright but the image on a monitor 35 is as shown by 40. Consequently, the image on the monitor 35 is observed to discriminate the direction in which the prism 1 is deviated from the prescribed position out of X, Y, and θ directions, and devices 80 and 81 are properly driven. Thus, the positional deviation of the attaching position is detected.