An FFT analysis is performed to the image data of one frame which has been decoded, and a cutoff frequency on the assumption that the original signal has first-order attenuation characteristics is obtained each in the horizontal direction and in the vertical direction. A sampled-data H∞ filter (digital filter) which has been previously designed in such a manner as to have different parameters corresponding to different cutoff frequencies is selected. By using this filter, a filtering process is performed to the image data which has been decoded both in the horizontal direction and in the vertical direction. Since the variety of the analog frequency characteristics of the original image is taken into account for every frame of image, the information that seems to be a noise yet actually exists in the original image is less likely to be removed, and a mosquito noise and block noise associated with a coding/decoding process can be efficiently removed.