terminals 206 and 302 and provides at an output terminal 403 the inter block difference between the block 0 in the picture memory 2 and the block 0b in the background memory 3 reference numeral 43 identifies a comparison decision circuit for comparing the inter block differences b in terms of magnitude the number of such circuits 43 necessary for comparing all the inter block differences with one another is the number of blocks minus one but in fig8 only one of them is shown in the comparison decision circuit 43 reference numeral 431 indicates a substractor for calculating the difference between two inter block differences 432 a plus minus decision circuit for applying an on off control signal to the succeeding switching circuit 433 depending upon whether the difference is plus or minus and 433 a switch for selecting the smaller inter block difference in accordance with the control signal further the result of decision by the plus minus decision circuit 432 is provided to the switch 433 for selecting the smallest block number sequentially selecting block numbers which provide the smaller inter block differences by combining l 1 pairs of such comparison decision circuits 43 and selectors 44 the value of the smallest inter block difference b 0 39 n is finally obtained at the output 402 and the block number n at that time is derived at the output 401 fig9 is a diagram illustrating an example of the arrangement of the background detector 5 in fig4 in fig9 reference numerals.