To provide a monitor system for a communication system between a master station and slave stations where all the slave stations inform the master station about their states, collective processing is eliminated, the increase in the communication load on the master station is prevented and the communication cost can be reduced.

The communication system is provided with a master station 1 and slave stations 2, a ring shaped network 4 formed by software processing is configured between the slave stations 2, and the master station 1 dispatches a mobile agent 5 to the slave stations 2 to monitor the functions of the slave stations 2.