The image input from the image inputting portion 10 are DCT-converted by the DCT portion 20, then the converted result is quantized by the quantizing portion 40, then the quantized result is entropy-coded by the entropy coding portion 50, and then the coded output is output. The quantization table calculating portion 30 holds respective reference quantizing steps for the reference resolution, then receives the resolution data of the input image, then calculates respective quantizing steps based on (quantizing step)=(reference quantizing step)/(reference resolution)(resolution of the input image) so as to fit to the input image, and then prepares the quantization table.