PURPOSE: To prevent the reduction of throughput by hierarchically connecting a network monitor device and multiplex transmission equipments and performing polling in parallel by the network monitor device and multiplex transmission equipments.

CONSTITUTION: A network monitor device 1 is connected to a lower-order network by a line 3 to monitor lower order multiplex transmission equipments 2, and multiplex transmission equipments 2 monitored by the network monitor device 1 monitor more lower-order device groups. Information from lower-order device groups are gathered by the higher-order multiplex transmission equipment 2, and these information are transferred to the more higher-order multiplex transmission equipment 2 at the time of polling from the equipment. Since the network is hierarchically constituted and hierarchically monitored, internal monitor and control are performed in parallel with each hierarchy as the unit even at the time of the increase of the number of devices in the network. Thus, the throughput is not reduced.