PURPOSE: To set up an effective visual field automatically even if the position or direction of an image pickup device is slightly changed by arranging marks indicating the limit of the effective area and detecting the marks from a picture of the marks and an object on the image pickup side to determine the coordinates of the effective visual field limit.

CONSTITUTION: The marks MK which are cylindrical white substances are arranged on both the sides of a conveyer C and a picture I of the object OJ and the marks MS is formed on the image pickup device 10 through a lens 10a and scanned by a scan driving circuit 11. The marks MK are detected by a pre-processing circuit 12 and informed to a terminal end determining means 20. The circuit 20 sets the adoption of any one of the outer edge and inner edge of the mark substances. A limit setting means 40 reads out the output of the driving circuit 11 on the basis of the position signal to determine the coordinate value of the limit position. A signal extracting means 40 separates a partial signal string VSP corresponding to the effective area from a video signal data string VSD on the basis of said coordinate value. Instead of said reflection type, a light shielding type can be also adopted.