an odd numbered row detection memory 16 od if the line is an odd numbered row by the soft switch sw and likewise when the signal starts to rise from the point min 0 and a rise by the displacement level w is detected the data of this point as a minimum value min 0 and the position minp 0 of the detected pixel are written in the same manner thus successively respective data of the maximum value max 1 minimum value min 1 maximum value max 2 minimum value min 2 maximum value max 3 minimum value min 3 maximum value max 4 minimum value min 4 and maximum value max 5 and the respective positions maxp 1 5 and minp 1 4 not shown of the detected pixels are written into an even numbered row detection memory 16 ev if the current horizontal scanning line is an even numbered row or an odd numbered row detection memory 16 od if the line is an odd numbered row in the illustration as shown by the level a if the displacement width is less than the displacement level w the displacement starting point is not regarded as a maximum or minimum value point see the displacement level setting 14 a of fig1 the second processing is carried out as shown in fig2 in step s 20 it is judged whether.