Microwave Power Transmission (MPT) technology is one of the most essential parts for Solar Power Station/Satellite (SPS). We study on application of magnetrons as DC-RF converters for the MPT transmitting system. Magnetrons cost much cheaper, have much higher DC-RF efficiency over 70% and much lighter system weight per 1 watt RF output than semiconductor amplifiers although they have wider bandwidth of the fundamental frequency and spurious noises in various frequencies. Spurious noises are radiated from the transmitting system and interfere in the other communication systems both in space and on the Earth. The objective of this study is the improvement of the spurious noises generated from magnetrons. Experimentally, magnetrons driven by DC stabilized power supply had not only narrower bandwidth of the fundamental frequency but also lower spurious noise levels when filament current is turned off than when it is turned on. Some spurious noises are probably caused by the intermodulation between the low frequency spurious noises, which frequency is below 1 GHz, and the fundamental or the harmonics. We also verified that the harmonics levels of the measured magnetron in our measurement system were below -70 dBc, which are comparable to or better than those of some semiconductor amplifiers, and that the harmonics were not improved greatly when the filament current was turned off because the source of the harmonics is the distortion of the fundamental.