A power supply for a magnetron provided with an inverter circuit, a high-tension transformer, a magnetron. In order to shorten the time required from the feeding of the power to the cathode of the magnetron to the arrival at the rated output through the start of the oscillation, a mechanism for detecting the oscillation start time point of the magnetron is provided so that the controlling operation is adapted to be effected to be rated output immediately after the oscillation. Winding for output voltage detecting use is provided on the high-tension transformer to detect the oscillation of the magnetron.
