PURPOSE: To improve the detecting accuracy of a center by, when a cursor center is not in the center circle of the binary image of a target mark displayed on a monitor, investigating the gradation of the binary image in fixed process in the directions of X, Y axes to severally obtain the starting' point of the gradation in the directions of X, Y axes, and comparing the distance of a starting point with diametral reference value and fitting it to the above value.

CONSTITUTION: In the case where a cursor center 13 is not in the center circle of the binary image of a target mark (b) displayed on a monitor 3, an image is radially investigated within an investigation reference point set up on the monitor 3 in fixed order at each fixed angle to obtain a starting point W being the same in gradation as the center circle (b1). The inside of a gradation part is investigated from the starting point W in the direction of X axis to obtain starting points X1, X2 being different in the gradation from the center circle (b1). Investigation in the direction of Y axis is carried out about the midpoint coordinate positions of the starting points X1, X2 to similarly decide starting points Y1, Y2 in the direction of Y axis. Starting points X'1, X'2 are similarly decided about the midpoint coordinate positions of the starting points Y1, T2. intersection point O of both the starting points is recognized as the target mark (b).