High-speed power line communication (PLC) systems have been developed for home networks. However, the transmission characteristics of an indoor AC mains line have not been clarified. We studied the influence of the appliance operation state on the transmission characteristics of the indoor AC mains line. The AC mains line and appliances were represented by four-port networks and two-port networks. The transmission characteristics were calculated by a model constructed with these networks. The two-port network parameters of the appliances were measured in both operating and stop states. The equivalent circuits were determined for both states based on the condition that the frequency characteristics of the circuits agreed with the measured value. A simple AC mains system was constructed and the transmission characteristics were measured to compare with the calculated value in various states. The results indicate that the calculated value agreed well with the measured one, and the transmission loss deviation resulting from the appliance state was within 10 dB. A common mode choke coil was inserted between the AC mains line and the appliances to reduce the transmission loss deviation resulting from different states. The measured and calculated values indicate that the deviation was reduced from 10 dB to 0.5 dB by inserting the filter.