To provide a communication system which can select optionally its desired slave station via an external monitor and can monitor this slave station.

This communication system comprises a master station 101, N pieces of slave stations 111-11N and a monitor station 131. The axis selection switch of an optional slave station is selected and set at an axis address in communications with the station 131 among those axis selection switches 121-12N attached to the stations 111-11N respectively, so that the slave station is monitored by the station 131. Thus, it is possible to select optionally one of external slave stations 131 during operation via an operating monitor and to monitor this selected slave station. It is also possible to stop the monitoring operation of the slave station monitored by the station 131 to secure a monitoring deactivated state.