A unitized wiring portion (wiring unit) has a hard pipe member, a wire provided in the pipe member, and outside wiring connectors connected at each end of the wire. A plurality of wiring units are prepared in advance, wherein wiring units are divided into multiple groups having different length of wires and so on. The wiring units are arranged in a structure (e.g. a vehicle body) and the outside wiring connectors are connected to each other to form a wiring network. The wiring units are fixed to frame and/or reinforcing members to thereby increase mechanical strength of the structure.