Noise of ball bearing can be divided into two cases ; one is the case of a simple ball bearing, the other is the case of a fitted one. The noise of the latter case has not been researched systematically. The author measured the noise of three kinds of the fitted ball bearings, analyzed the peak frequency in the frequency spectrums, and obtained the results as follows : (1) The noise of a fitted ball bearing has the frequencies above 1 kc/sec. The frequencies of the noise do not change at many shaft speeds, but the sound pressure level increases with the shaft speed. (2) The principal noises of a fitted ball bearing are due to the vibrations on the race rings, and the other noises are due to the vibrations on the cage. (3) The fitted ball bearing noise and the simple one are identical in essence.