The invention relates to a power supply device and a communication system. The power supply device supplies power to a main device. The communication system is provided with the main device mounting the power supply device therein and an external device connected to the main device. A feature of the invention is to offer a power supply device and a communication system that are capable of being responsive to external signals for returning to a normal operation mode with minimum power consumption in a power-saving operation mode. In the power saving operation mode, a main power supply circuit (60) is turned off and only an auxiliary power supply circuit (50) is operated. In order to detect external signal, the auxiliary power supply circuit (50) supplies power not to an entire interface section (20), but to a ring detection circuit (31), a LAN signal detection circuit (32), a 1284 signal detection circuit (33), a USB signal detection circuit (34), and a panel signal detection circuit (35) that are respectively provided with a minimum functions for detecting external signal.