The magnetron, mainly used for microwave heating, has the attractive features of high efficiency, high power, and low cost, but it generates noise at various frequencies. Hence, a filter must be inserted to suppress noise leakage to acceptable values. Also, since the magnetron has a low weight per unit output, this device is promising as a microwave power source for microwave power transmission, represented by the Solar Power Satellite concept. There is a danger that noise may affect other communications devices. The objective of the present research is to find the noise source of the magnetron, so that a low-noise magnetron can eventually be developed. In this paper, a time domain analysis of the noise generated from a magnetron driven by a half wave voltage doubler is carried out. The results confirm frequency pushing of the fundamental spectrum. With regard to the noise, it is confirmed that the harmonics are generated in synchrony with the fundamental. Also, the noise is subdivided into noise synchronized with electron circulation, low-frequency line noise, and high-frequency spurious noise generated in the region of transition of the anode voltage from constant to the nonconstant region, a noise group dependent on the frequency of 400 MHz, and low-frequency line noise at frequencies below 100 MHz generated at anode currents of less than 0.2 A.