A base unit suitable to improve easy exchangeability of a function unit in a dual wiring system is provided. The base unit is mounted in a wall surface of a structure, and connected to both of an electric power line and an information line previously installed in the structure. The base unit has a module port, which is detachably connected to a module connector of the function unit to simultaneously establish both of supplying the electric power from the base unit to the function unit, and making a signal transmission between the base unit and the function unit, thereby enabling the function unit to provide at least one of functions for supplying electric power from the electric power line, outputting information from the information line and inputting information into the information line when connected with the base unit.