To provide a code string forming apparatus which reduces the expansion time taken for displaying code data, i.e., quickly scales down and displays high-definition picture data coded as still pictures or moving pictures composed of continued still pictures and forms a code string of a still picture effectively expandable according to the designation of the expanding picture area, color components or expanding operation order.

The apparatus comprises a means 11 for inputting a code string expandable by a still picture expander 15, a means 12 for interpreting the syntax of the code string, a means 13 for forming a separate (new) code string expandable by the still picture expander 15 from that code string based on the interpreted syntax, and a means 14 for outputting the formed (generated) separate code string. The code forming means 13 forms a new code string by deleting or rearranging the code string.