Block noises generated on an input video signal that has been coded and decoded per pixel block are detected. The input video signal is differentiated per pixel to obtain a differentiated signal. Impulses of the differentiated signal are detected to obtain a detection signal carrying the impulses. The detection signal is integrated and compared with a reference signal to determine whether the block noise is generated on the input video signal. For noise reduction, the detection signal is filtered to obtain a corrected signal. The input video signal is delayed by a predetermined period. The correction signal is added to the delayed video signal to cancel the difference in signal level on the boundary between a first pixel block on which a block noise is generated and a second block adjacent to the first pixel block of the input video signal.