To provide a block noise detective method and device, a block noise reducing method and device. A pixel boundary difference detector detects a pixel boundary difference across a pixel boundary on thebasis of a difference value between pixel values of adjacent pixels across the pixel boundary on a picture plane of an input image frame, and a predicted pixel value on the pixel boundary predicted from at least a plurality of pixels on one side of the pixel boundary, an accumulator accumulates the pixel boundary differences for one image frame, and a block noise detector detects a block noise onthe basis of a result of the accumulation. The accuracy of block noise detection is improved, the effect of block noise reduction is improved, and the picture quality is improved.