PURPOSE: To hold a part of a position detection system in common along with an anterior eye part observation system and to facilitate constitution by receiving the reflected luminous flux due to the cornea within the predetermined area of the anterior eye part observation system and storing the image in the area as a video signal to calculate the shift of the alignment position of an eye to be examined.

CONSTITUTION: The reflected luminous flux of a light source due to the cornea Ec of an eye E to be examined is reflected by a half mirror 2 and projected on an imaging element 5 as a cornea reflected image M1 through a lens 3. The reflected luminous flux due to an anterior eye part is projected as an anterior eye part image M2 in the same way. Only the image in the area A preset on the imaging element 5 is outputted to a signal processor 6 as a video signal. When the alignment of the eye E to be examined is proper, a sharp rectangular peak is generated at the middle point of two crossing points R1, R2 of the boundary line of the area A and a horizontal center line (l) and, since the peak becomes blurred when the peak is shifted in the direction along an optical axis 01, the three-dimensional shift of the eye E to be examined can be calculated from the position of the peak and the magnitude relation between the peak value and the threshold value to the level thereof by a signal processor 6.