Wavelet transformation is performed at a plurality of levels as to image signals. A first buffer stores, independently for each of the levels, a part of coefficients from results of analysis filtering in a horizontal direction of the image signals. A second buffer stores, independently for each of the levels, a part of coefficients generated in a computation process of analysis filtering in a vertical direction of the image signals. A vertical filtering unit performs the vertical direction analysis filtering, using coefficients read out from the first and second buffers. A horizontal filtering unit performs analysis filtering in the horizontal direction, using coefficients from the results of the vertical direction analysis filtering as the input of the horizontal direction analysis filtering for the next level, except for the final level of the levels.