PURPOSE: To detect a shift amt. with high accuracy without increasing the scale and time of calculation by forming the light image of a subject on two photoelectric element arrays and determining the shift amt. when a correlation amt. is max. by the prescribed calculation while shifting both output patterns by a prescribed amt.

CONSTITUTION: The luminous flux past image forming optical system 1 passes through the light transmission region 2a of a field lens 2, is reflected by a pair of concave mirrors 4, 5 and mirrors 6, 7 in a block 3 and forms the light image of the subject on photoelectric element arrats 9, 10. The relative positions of the two light images change according to the focusing condition of the system 1. A correlation amt. C(L) is calculated from the output data (ai), (bj) (i=qWr, j-i= L) of the arrays 9, 10 in an arithmetic part 14 for the correlation. When a discriminating part 16 judges that C0 is min. from the three correlation amts. C-1, C0, C+1 with respect to the shift amts. L-1, L, L+1, the shift amt. that gives the max. correlation from the point across the axis L of abscissa is determined by an intrpolation method in an interpolation part 17. The shift amt. is thus obtd. with high accuracy.