To grasp the communication traffic state in a network and to quickly deal with the fault by storing a logical path information-based flag of the frame that is read out of the passing user data and then reading out the flag information to confirm the data flow of each communication path.

In a communication network constituted of the switchboards 1a to 1d serving as the data communication devices of logical multiplex systems connected to a transmission line, each frame processing part 10 of data communication devices 1a to 1d receives a frame relay signal FR of the passing user data. A frame address read control part 12 reads the logical path information on the frames and stores flags which are set for each logical path information in a flag region 130 of a storage 13. Then a read processing part 14 reads the flag states out of the storage 13. Thus, the data flow is confirmed for each communication path of the communication network, based on the flag information read out of the storage 13.