PURPOSE: To provide a data compression/expansion device capable of compressing and expanding data at a high speed with a small circuit scale.

CONSTITUTION: This data compression/expansion device is constituted of a parallel/serial conversion circuit 61 for converting parallel data read from a data storage device to serial data, an intermediate data storage device 63 for tentatively storing the serial data capable of outputting them either as serial or parallel data and a data conversion operation circuit 62 for executing a data conversion operation in the form of a serial operation to serial input from a parallel/serial conversion operation circuit 61 and an intermediate data storage circuit 63. The data conversion operation circuit 62 parallelly arranges serial circuits 62a and 62b for multiplying different coefficients and serial conversion output is obtained by accumulating multiplied results by the parallelly arranged serial circuits 62a and 62b by the intermediate data storage device 63.