PURPOSE: To accurately detect the position of a target object by detecting an image area, where one value preset among binarized image is continued from the edge part of a screen, and inverting the value of image data in the detected image area.

CONSTITUTION: The image data for one screen inputted from a TV camera are stored in a frame memory 11. The image data are successively read from the frame memory 11 and binarized by a binarizing circuit 12. The binarized image data are temporarily stored in a first image memory 13. Then, the binarized image data are read from the first image memory 13 to detect the image area where either preset one value in white or black is present continuously from the edge part of a rectangular area by first and second connection converting parts 14 and 15. Thereby, the value of image data in the detected image area is inverted and stored in a fourth image memory 19. Thus, the position of the target object can accurately be detected.