A monitoring apparatus and apparatuses to be monitored each have an address holding unit which holds the address of a group to which each apparatus belongs. A data frame to be transmitted between the monitoring apparatus and each of the apparatuses to be monitored includes an identifier which specifies communication between two apparatuses or group broadcast. When the identifier specifies the group broadcast, a group address is put in the data frame. In this manner, a large number of apparatuses which form a large-scale network can be divided into groups, and the group broadcast can be carried out when a plurality of monitoring apparatuses are used for monitoring the apparatuses, thereby eliminating the need of polling.