A power supply system which is designed to control the power supply to, for example, a building-block private branch exchange (PBX) system is provided which includes main and extension cabinets having disposed therein AC power supply units, respectively, and a power supply control circuit. The main and extension cabinets are supplied with AC power from an external power source and provide DC outputs. The power supply control circuit includes a first and a second switching circuit. The first switching circuit allows the supply of AC power to the AC power supply unit of the extension cabinet when the power supply system is turned on and interrupts it when the power supply system is turned off. The second switching circuit establishes communication between the AC power supply unit of the extension cabinet and a backup battery when the power supply system is turned on and blocks it when the power supply system is turned off.