A dual wiring system with improved function expandability is provided. A 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. Each of function units has at least one of functions for providing 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. The base unit is detachably connected with one of the function units by use of a joining member, so that electric power is supplied from the base unit to the function unit through a power transmission means, and a signal transmission between the base unit and the function unit is obtained through a signal transmission means.