A power supply apparatus for supplying predetermined supply voltages respectively to an anode electrode, a cathode electrode, a collector electrode, and a helix of an electron tube. The power supply apparatus comprises an anode switch for turning on/off the anode voltage output, and an anode switch control circuit for controlling the on/off operation of the anode switch such that a pulsed anode voltage is repeatedly applied to the anode electrode a plurality of times at a predetermined period when operation of a helix power supply and a collector power supply is stopped.