Abstract: The network system of the present invention is comprises lower segments constituted in each of a plurality of organizations, floors, offices, and an upper segment that connects the lower segments together. The network system comprises an improved L2 switch that is installed in each lower segment and has a function to transmit a frame, which is transmitted from other network devices of each lower segment to the improved L2 switch itself to a center device that is installed in upper segment; and the center device that is individually connected to the improved L2 switch and controls the data communication of the frame between the improved L2 switches. According to the present invention, a network system; wherein data communication can be performed easily between the lower segments, and an ideal data transfer system which can be prepared optionally by changing the center device characteristics when necessary, can be offered.