To provide a high-speed/high quality data transmission system employing a power line and a network system employing the system.

An end of an optical fiber 10 is provided with a photoelectric converter 110, a 1st modem 120, and a branching filter 150. Further, a customer is provided with a branching filter 250 and a 2nd modem 210. Then the branching filters 150, 250 are connected by a power branch line 16 and an outdoor power line 14. In the case of communication, both the modems 120, 210 are used to convert data to have a frequency of 10-30 MHz on the power branch line 16 and the outdoor power line 14 and transmit the data by a spread spectrum communication system with a high frequency signal with e.g. GHz order. Thus, high-speed/high quality data transmission is attained and customers are placed on the outdoor power line 14 after the branch filter 150 in a multi-drop form. Thus, a novel network system employing the power line can be built up.