The objective of the present study is an experimental research and development on noise reduction of current commercial magnetrons, which will be applied to a SPS. We have found out that turning off the filament current of a magnetron worked exceedingly well for reducing the levels of spurious noises as well as narrowing the oscillation bandwidth when a magnetron was driven by DC stabilized power supply. We have experimentally verified that these improvements are produced by the smoothing effect of anode current, the reduction of thermal effects, and the decline in the number of thermal electrons in the vicinity of both filament ends. Next, we found out that spurious noises were reduced when the rated anode voltage was decreased or the anode-cathode distance was increased. Then, we evaluated effects on SPS transmitters through experimental results.