A block noise detector has a spatial difference calculator that calculates differences between values of adjacent pixels. A spatial difference comparator detects edges by comparing the calculated differences. Eight counters count edges detected at different groups of positions spaced eight pixels apart in each horizontal line. The maximum count and the group of positions at which it occurs are detected at the end of each horizontal line to detect block noise and the positions of the block boundaries. The block noise detector is small in size because it only has to count edges in one line at a time, and detects edges accurately by comparing the difference at a given position separately with differences to the left and differences to the right.