In a microwave power source apparatus for a microwave oscillator for generating a microwave, a DC power source supplies a DC power for generating the microwave to the microwave oscillator. A measuring section of a voltage standing wave detector is mounted on a microwave transmission line connected between the microwave oscillator and a microwave load, and measures either one of an impedance seen looking toward the microwave load at a mounted point thereof and a reflection coefficient thereat by detecting a voltage standing wave of the microwave being generated by the microwave oscillator and propagating on the microwave transmission line. A controller calculates a progressive wave power of the microwave propagating on the microwave transmission line from the microwave oscillator toward the microwave load based on either one of the impedance and the reflection coefficient measured by the measuring section, and controls the DC power source to adjust the progressive wave power of the microwave to a predetermined desirable adjustment value based on the calculated progressive wave power.