PURPOSE: To obtain an accurate binary coded signal by setting the threshold level between the white and black peak levels most approximate to the analog picture signal to be binary coded.

CONSTITUTION: The analog picture signal supplied from a terminal 60 is converted 64 into a digital signal and then written to memories 67 and 68 when the white and black peaks are detected by a black peak detecting circuit 63 respectively. Then the black and white peak levels are read out of the memories 68 and 67 when the white and black peaks are detected out of a delayed 61 analog picture signal by white and black peak detecting circuits 65 and 66 respectively. These peak levels are held until the next white and black peaks are detected. The white and black peak levels read out of the memories 67 and 68 are added 69 together and then converted into analog signals by a D/A converting circuit 70 with a 1/2 conversion factor, for example. In a comparator 71, the delayed 61 picture signal is binary coded with the output of the circuit 70 defined as the threshold level.