Control device for a power over Ethernet system having multiple power source devices comprises plural control circuits and a signal bus connecting them. Each control device connects plural power source devices and plural port switches, which controls power supply to a port to be connected by a power consuming device. Detection circuit detects at least one power supply state combination. Control signal generator picks up a power control combination signal corresponding to a detected power supply state combination from a power supply to power control look-up-table, upon change in the power supply state and provides the power control signals to corresponding port switches.