A dual wiring system with easy exchangeability of a function unit. A gate device 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 function unit has 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 gate device. The function unit has a module connector including an electric power connector and an information signal connector, which is detachably connected to a module port of the gate device comprised of an electric power port and an information signal port. To further improve function expandability, an additional function unit can be detachably connected to the function unit.