To reduce a delay time caused between terminals in the case of relaying of a PBX and to especially eliminate a cell assembling delay, a fluctuation absorption delay, a voice coding/decoding delay and a relay exchange transmission delay.

A cell multiplex/demultiplex section 102 has a cell multiplex/ demultiplex function that multiplexes cells received from a plurality of channel sections 200, 250, and demultiplexes and gives cells to a plurality of the channel section 200, 250, a time division multiplex/demultiplex section 301 has a data multiplex/demultiplex function that applies time division multiplex/demultiplex to digital data received from a plurality of the channel sections, each of the channel sections has demultiplexed cell passing sections 207, 208, cell passing sections 201, 202, a 1st passing section 205 that replaces a header of a cell not passing through the demultiplexed cell passing sections 207, 208 with other header to give the resulting cell to an ATM common section 100, and a 2nd passing section that receives cells coming from the cell multiplex/demultiplex section 102 and not passing through the demultiplexed cell passing sections 207, 208 to give the cells to a TDM common section 300 so as to decrease a delay time caused between terminals in the case of a relay operation of a PBX.