PURPOSE: To eliminate the torsional component of oscillation and to prevent the lowering of efficiency and generation of sound by forming the circular part of an oscillator into a shape differing in the flexural rigidity between the inside and outside of said circular part so as to reduce the torsional component in the linear part to be used for driving.

CONSTITUTION: The circular part of an oscillator is formed into a shape with a difference in the height of a cross section, where the height of the inside is lower than that of the outside and the flexural rigidity of the inside is smaller than that of the outside. Also, said circular part is formed into a shape differing in the flexural rigidity of its inside (inner diameter) and outside (outer diameter) and capable of raising the rigidity of the linear part. Thus, it is possible to eliminate the torsional component other than that in the oscillation mode required for driving and to prevent the lowering of efficiency and generation of sound.