PURPOSE: To detect the center position of a circular pattern having a projection by adding/subtracting the radius value of a smaller diameter on X, Y coordinate axes to the coordinates of both the ends of a larger diameter and comparing these three values to determine the center coordinates.

CONSTITUTION: A circular object 1 having a small projection is read out through laster scanning by an image pickup device 2. The image pickup output from the image pickup device 2 is converted into a binary signal by a binary circuit 3. A feature extracting circuit 4 extracts the coordinate position of the charged point of the binary signal and a data processor 6 operates on the basis of said data. When the diameter of X(Y) axis direction is larger than that of the Y(X) axis direction, the radius of Y(X) axis direction is calculated. The radius is added/subtracted to each of the minimum value and maximum value of the X(Y) coordinate to find out two false center coordinates and any one of the false center coordinates when coinciding with the previously found Y(X) coordinate is determined as the center position of the X(Y) axis direction.