To prevent degradation of communication quality by suppressing degradation of balancing of a transmission path caused by installation of a switch in one-side wire line of a branch power line, in power-line carrier communication using an indoor power line.

An opening-closing switch SW for switching turning-on/off of current between two terminals is interposed in an intermediate part on one side of a pair of wire lines WL1, WL2 of a branch power line BL3; and, when a line length from one-side core power line ML of the pair of wire lines to a power load LD is larger than that on the other side when the opening-closing switch SW is on, a balancing element 3 for suppressing a phase difference due to a line difference between the pair of wire lines WL1, WL2 at a carrier frequency used for power line carrier communication is serially inserted on the other side of the pair of wire lines WL1, WL2.