1. A block noise reduction device for reducing block noise from a decoded image signal obtained by decoding a compressed and coded image signal, comprising: a frame correlation determination part for determining a level of correlation of the decoded image signals between frames; a flat region detection part for detecting an image region in which a difference in brightness between neighboring pixels is small as a flat region from the decoded image signal; a high-frequency region detection part for detecting an image region including a high spatial frequency component as a high-frequency region from the decoded image signal; a smoothing processing part for performing smoothing processing to the decoded image signal corresponding to a predetermined region; and a processing region setting part for specifying an image region in which the smoothing processing part is to perform smoothing processing to the decoded image signal, wherein the processing region setting part sets at least a region in frames in which a correlation between frames is low and which is a flat region and not a high frequency region in the decoded image signal, as a processing region.