A power supply apparatus is provided, that supplies a bus voltage VBUS to a detachable device. A DC/DC converter generates a DC voltage VOUT used as the bus voltage VBUS. A USB controller receives the DC voltage VOUT as a power supply voltage, and detects whether or not the device is connected. The DC/DC converter generates the DC voltage VOUT such that (i) when the device is detected, the DC voltage VOUT has a specified voltage level, and such that (ii) when the device is not detected, the DC voltage VOUT has a voltage level that is lower than the specified voltage level and that is higher than a minimum operating voltage level required to operate the USB controller.