A battery is disposed at the portion beneath the rear seat of a vehicle. The positive electrode of the battery is connected through a power bus for power supply to a rear junction block (R-JB). A rear integration module (RIM) can be coupled to the R-JB through a plug-in operation. The positive electrode of the battery is also connected through a power bus to an under dash junction block (UD-JB). A body control module (BCM) can be coupled to the UD-JB through a plug-in operation. The power bus connection terminal of the UD-JB is connected to a fuse relay block (F-JB) through a power bus. A front integration module (FIM) can be coupled to the F-JB through a plug-in operation. The power bus connection terminal of the F-JB is connected to a starter motor through a power supply line and further connected to an alternator through a power supply line from the starter.