PURPOSE: To ensure the highly accurate measurement for a two-dimensional image pickup device by reducing the effect of the lens distortional aberration of the image pickup device.

CONSTITUTION: A master work including four rectilinear parts is photographed by a TV camera, and the image data E1, E2, E3 and E4 on these rectilinear parts are extracted out of the image data on the master work. Then the curvature circles C1, C2, C3 and C4 are acquired for the data E1-E4, and the straight lines L1 and L2 passing the intersecting points P1, P2, P3 and P4 of two pairs of curvature circles are acquired. The intersecting point C between both lines L1 and L2 is defined as the center of the lens of the TV camera. Then a distortion correction coefficient is calculated from the center coordinates of the lens and the coordinates of three points set on a curvature circle. Thus the distortion factor of the lens is corrected. Then the coordinate value of an optional point is corrected based on the lens center coordinates and the curvature correction coefficient.