PURPOSE: To facilitate FDM.TDM conversion by performing conversion processing separately in plural stages after an FDM signal is converted, and thinning out a sample signal in every stage and branching it into two in the next stage.

CONSTITUTION: The FDM signal is converted by an A/D converter 12, etc., into a digital signal and the following conversion processing is performed separately in plural stages; and the sample signal is thinned out in every stage by digital filters 13a and 13b, and 15a∼15d and branched into two in the next stage, and they are processed by arithmetic circuits 14, and 16a and 16b. This is repeated to obtain signals corresponding to channels, and those signals are arrayed in a prescribed time slot to obtain a TDM signal. Therefore, the frequency of branched processing increases toward the final stage, but a sampling frequency is becomes lower, so the amount of arithmetic in every stage is equal.