A motion detection signal delivered from a mixer is output through a series circuit of a maximum value calculation portion and a minimum value calculation portion. In each of the calculation portions, calculation is performed within a block of 3-dot.times.3-line. Missing of motion detection is prevented from occurring in the maximum value calculation portion because the motion detection signal of the pixel under attention is calculated therein with the values within the block and, accordingly, the motion detection signal is expanded in the horizontal and vertical directions. A detection spread to a still picture portion occurring in the calculation portion is suppressed in the minimum value calculation portion because, when surrounding motion detection signals therein are lower than the motion detection signal of the pixel under attention, the motion detection signal of the pixel under attention is replaced with the lower value. With the described arrangement, the missing of motion detection is advantageously prevented without inviting the detection spread to a still picture portion.