PURPOSE: To reduce the memory capacity for a data storing device which expands and restores the compressed data on the color static images.

CONSTITUTION: A decoding circuit 11 outputs and stores the data obtained by adding a single bit serving also as an underflow/overflow flag to the decoding data having the prescribed number of bits into a memory 12. Therefore the bit with of the memory 12 can be reduced by one bit compared with a conventional case where a single bit is required exclusively for each of underflow and overflow flags.