A power supply circuit for supplying electric power to a portable computer includes a main battery, a first control circuit, a backup battery and a second control circuit. The main battery includes a first output terminal and a second output terminal shorter than the first output terminal. When the main battery is drawn out of the portable computer, a disconnection between the second output terminal and the portable computer occurs earlier than a disconnection between the first output terminal and the portable computer to from a time difference between the two disconnections. During this time difference, the main battery is controlled by the first control circuit to change from supplying electric power to the portable computer to not supplying electric power to the portable computer, and the backup battery is controlled by the second control circuit to supply electric power to the portable computer instead of the main battery.