A large number of relays are concentrated in a limited space to utilize the space effectively. A relay box containing a large number of relays can be disposed in a narrow space between a rear fender and a seat bottom plate. The relays are mounted on three substrates within the relay box. Plural relays are arranged in a line on each of the substrates and the substrates are arranged in a stepped fashion like stairs. Therefore, the relay box is substantially in the shape of a parallelogram, and thus can be disposed in a narrow space along the rear fender of a vehicle. Conductors of the relays are drawn out along the rear fender.