When a power switch is turned on, first and second analog switches are turned on while a third analog switch is turned off. Accordingly, at the time of starting, a forced excitation driving circuit supplies to a vibrator an oscillation output pulse which is substantially at the resonance frequency of the vibrator, thereby forcibly driving the vibrator in an excitation manner. Thereafter, when the vibrator substantially attains its stationary state and thereby the charged voltage of a capacitor reaches a threshold value of an inverter, the first and second analog switches are turned off while the third analog switch is turned on. Accordingly, the output of a self-excitation circuit is supplied to the electrode of the vibrator so as to vibrate the vibrator in a self-excitation manner.