This invention allows Huffman encoding using a common Huffman table according to basic quantization values Qi,j even when image data expressed by n bits falling within the range from L to K is JPEG-coded, and can suppress the Huffman table size from increasing. To this end, a basic quantization table storage unit stores quantization step values Q0,0 to Q7,7 used in baseline JPEG coding. A minimum quantization step generator outputs a minimum quantization step value Qn—min to a comparator/selector according to the number n of bits of each color component of image data to be coded. The comparator/selector compares the quantization step values Q0,0 to Q7,7 with Qn—min and selects larger ones, and outputs the comparison results to a quantizer as Q′i,j. The quantizer stores Q′i,j in a quantization table storage unit and quantizes orthogonal transformation coefficients output from a DCT transformer.