A power supply apparatus supplies regulated power to an external apparatus. A power switch is turned on to receive AC power and turned off not to receive the AC power. The AC power is rectified by a rectifying section and is switched by a switching section into switched DC power which is smoothed by a rectifying/smoothing section. Upon reception of an alarm signal, the power disconnecting section stops sending the switched DC power to the rectifying/smoothing section. If the AC switch is turned off and then back on again after stopping sending the switched DC power to the smoothing section, the power disconnecting section allows receiving of the AC power only a time after turn-off of the power switch. Upon reception of an auto-off signal indicative of an idle state of the external apparatus, an auto-off section does not send the switched DC power to the rectifying/smoothing section.