In order to increase the rotational speed of an ultrasonic motor using a flexural traveling wave, slits made on the surface of the stator are very effective. In this paper the projections made by the slits are called “sub-reeds” for convenience. Though the sub-reeds play important roles in an ultrasonic motor, their dimensions are generally decided through trial and error. This paper presents the optimum design method of a sub-reed by using elastic vibration analysis. This method calculates the dimensions of the sub-reed of which resonant frequency is same as that of the basic disc. This paper also shows that the resonance of the sub-reed mainly causes the magnification of the amplitude by using finite element analysis. The optimum design method and the mechanism of the magnification mentioned above are confirmed by experiments.