PURPOSE: To mix many channels with a small number of conductors by providing an analog adding means, an A/D converting means, and a digital adding means, and performing the analog addition of parallel input data of plural channels through buses so that corresponding bits are added.

CONSTITUTION: Bit output currents of channel modules 34-1-34-N are passed through buses 40.0-34.23 as analog adding means and the corresponding bits are added by analog. The respective current values after the analog addition are converted into voltage values, which are converted into digital signals by A/D converters 46.0-46.23. Then addition data of the respective bits is matched in their digits by a digital addition circuit 48 and are added mutually and respective bit signals of final digital mixing outputs are outputted from output terminals 50.0-50.23. Therefore, when the number of output bits of the A/D converter 46 is M, channel modules of 2M channels can be connected.