In a data decompressor, a variable length decoding unit extracts a variable length code from compressed image data, performs variable length decoding of the extracted variable length code, and restores quantized data before the variable length encoding was applied. An inverse quantization unit performs inverse quantization on the restored quantized data to generate orthogonal transform coefficients of each block and writes the generated orthogonal transform coefficients to a memory. An inverse orthogonal transform unit reads the orthogonal transform coefficients of each block from the memory and performs an inverse transform of the orthogonal transform performed by a data compressor on the orthogonal transform coefficients to thereby restore an image of each block. The inverse orthogonal transform unit is constructed so as to perform the inverse transform of the orthogonal transform coefficients in parallel with the variable length decoding of the variable length decoding unit and the inverse quantization of the inverse quantization unit.