A power supply apparatus for a traveling-wave tube disclosed herein eliminates the need for isolation through a vacuum relay or the like, and is therefore fabricated in small size and at low cost. An oscillator circuit generates an oscillating signal at a frequency optionally selected from a plurality of frequencies. An inverter is applied with the oscillating signal from the oscillator circuit to generate an AC voltage signal at the frequency of the oscillating signal. A transformer transforms the AC voltage signal generated by the inverter disposed on the primary side and supplies the resulting signal to the secondary side. A rectifier circuit, which is disposed on the secondary side, rectifies the AC voltage signal transformed by the transformer for application to the traveling-wave tube. A frequency detector circuit detects the frequency of the AC voltage signal applied from the transformer to the rectifier circuit to generate a device control signal in accordance with the frequency. A control device controls the application of a voltage to an anode electrode of the traveling-wave tube in response to the device control signal.