Owing to the difference of diameter between each ball in a bearing, the center of inner ring is slightly deviated from the axis of rotation of shaft. The balls revolve with a constant angular velocity ω1=α1ω round the center of inner ring, wherein ω is the angular velocity of shaft. The balls being rotated, the direction of this deviation of inner ring is revolved with the same angular velocity ω1. If the natural frequency of shaft is equal to ω1, this small deviation is sufficient to cause the critical speed to occur which has the circular frequency ω1. In common types of ball bearings, the value of 1/α1 is larger than 2, so this critical speed is higher than the major critical speed. The amplitudes of vibrations in this critical speed depend on the amount of the difference of ball diameters.