An SRAM is used as a pixel buffer which stores a partial image region copied from a frame buffer. A vertical filter reads a pair of target pixels arranged in a vertical direction from the pixel buffer and performs a filtering process of a discrete wavelet transform in the vertical direction. An intermediate result obtained in the filtering process is stored in a register. The vertical filter uses the intermediate result in filtering the next two pixels. A horizontal filter receives two transformed results from the vertical filter and performs a filtering process on the transformed results in the horizontal direction. The intermediate results obtained in the horizontal filtering are also stored in a register and utilized in the next filtering.