PURPOSE: To clear a memory provided in order to intermediate between shifting of index data, at a high speed and with a simple constitution.

CONSTITUTION: In the image data converter of a transmitting side, an index memory 13 is provided in a data bus 12 for connecting a discrete cosine converting part 10 and a variable length encoding part 11, and index data is shifted between the discrete cosine converting part and the variable length encoding part through the index memory. Also, an index position memory 14 is provided in order to realize a significant coefficient piece number type progressive system with satisfactory variable length code efficiency. A control part 15 controls both the memories, the discrete cosine converting part and the variable length encoding part to realizes the significant coefficient piece-number type progressive system. By utilizing a memory access constitution in this control part to the utmost, both the memories are cleared.