A multiplex communication system includes a plurality of networks and a data
relay
unit. Each node included in each network resets a relay flag which is a predetermined
bit of a data frame, when it sends the data frame to another node. If the data
frame requires to be relayed, the data relay unit receives and relays the data
frame. The data relay unit includes a relay flag check (RFC) section and a relay
flag setting (RFS) section. The RFC section checks whether the relay flag of the
data frame is set. If relay flag is set, the RFC section inhibits the data frame
from being relayed. If the relay flag is not set, the RFC section allows the data
frame to be relayed. The RFS section sets the relay flag of the data frame when
the data frame is relayed.