There are many researches on the noise of a ball bearing, and their conclusions about noise source can be divided into two main opinions ; one attributes it to elastic vibrations of the outer ring, the other to vibrations of the cage or ball. However, these conclusions are only based on a hypothesis. So, in the first place, the author measured the noises of three kinds of simple single row deep groove ball bearing, analyzed the peak values of their frequency spectrums, and obtained the results as follows : (1) The noise of a simple ball bearing is due to vibrations of the outer ring. (2) In the above vibrations, there are radial and axial bending ones which have the first and the second modes. (3) In the case of axial load, another vibration exists, and its vibratory system which was found by Mr. Tanaka, consists of a mass of outer ring and an axial stiffness of ball bearing.