A small size and light weight microwave power generating apparatus in which a power MOS FET which can withstand a high voltage and large current is combined with a microwave transmission circuit having a drain side coaxial resonating system having an internal conductor connected to the drain of the MOS FET to be tuned under a cavity parallel resonating condition to an oscillating frequency and a gate side coaxial resonating system having an internal conductor connected to the gate electrode of the MOS FET and an external conductor connected to the internal conductor of the gate side coaxial resonating system via a variable capacitor. A high frequency oscillation (100-1000 MHz) is realized without the use of a magnetron by the internal capacitances C.sub.GS and C.sub.DS of the MOS FET.