An improved drop/insert multiplexer for use in a channel access unit that uses common channel slots for data channels in first and second directions, and uses less wiring between a multiplexer/de-multiplexer section and a channel section. The drop/insert multiplexer has at the receive side thereof, a time slot phase difference setting stage for providing a predetermined phase difference between received timing signals as well as receive data from the first and second directions; a receive channel pulse generating stage generates multiplexed channels pulses; a receive channel pulse decoding stage and a PCM decoder reproduces a voice signal by receiving selected received data. At the send side thereof, a time slot phase difference correcting stage corrects the set phase difference for sending out channel data to the first and second directions; a send channel pulse generating stage generates multiplexed channel pulses; a send channel pulse decoding stage and a PCM coder produces PCM signals to send out to the first and second directions as channel data.