A composite system includes first and second devices, each operated using its own power source device as a driving source. The system comprises a signal line connecting loads of the first and second devices so that the first and second devices operate in cooperation with each other; and a power source line connecting a first line connecting the first power source device and the load of the first device via a diode and a second line connecting the second power source device and the load of the second device via a diode so that the first and second devices can operate using the power source device of the counterpart device as the drive source.