In coding through adaptive quantization after transforming an image into frequency areas, such as JPEG, quantization step width control information and other information are efficiently coded. An input image is processed in a blocking circuit and a quantizer. A quantization step width is determined by multiplying a matrix of a basic quantization table by an SF value of a scaling factor calculating circuit. Quantized AC components are coded in an AC component coding circuit 51. Quantized DC components are converted into DC differences and further into group numbers and additional bits. SF values are also converted into SF value differences and further into group numbers and additional bits. The DC components and the group numbers of the SF values are converted into two-dimensional Huffman codes. The above described code elements are multiplexed and outputted.