PURPOSE: To prevent quantized noise and saturation distortion, by outputting two sets of time division transmission codes detected at the same time to an output channel at called side at each sampling period alternately for temporary division transmission code each.

CONSTITUTION: When time division transmission codes S1, S2 and S3 are respectively inputted from input channels I1, I2 and I3 with the simultaneous communication of subscribers A, B and C, time division transmission codes S1+S2, S1+S3 and S2+S3 mixed in time division are respectively outputted from output channels O1, O2 and O3 and received with the subscribers C, B and A. Thus, the band of codes S1∼S3 inputted from the channels I1∼I3 is limited to a low frequency band at a prescribed value or below or remains unchanged, and outputted to the channels O1∼O3 singly or with being mixed in time division. Thus, the increase in quantized noise and saturation distortion can be prevented.