The power line communication (PLC) system should be investigated with respect to the influence on electromagnetic environments. Longitudinal conversion loss (LCL) and input impedance are important parameters for evaluating the influence because they are closely related to the radiated, conducted, and inducted emission. An indoor AC mains system consisting of electrical equipment and an AC mains line was modeled by four-port networks, and the LCL and the input impedance were calculated. The parameters of the four-port networks were determined from theory and measurement. The analytical model was examined using a simple network and the results show that the calculated values agreed with the measured ones. The LCL and the input impedance were investigated at the AC mains port in some existing buildings, and the measured results almost agreed with the calculated results derived from the indoor AC mains system model.