There are two types of piezo actuator structures: single-layer piezo actuators, which have electrodes only on the surface of the piezo ceramics, and multilayer piezo actuators, which also have electrodes inside the piezo ceramics. Since the maximum electric field strength applied to the piezo ceramics is 1 kV per mm, the single-layer piezo actuator requires an applied voltage of about 1 kV to maximize the displacement of the 1 mm thick piezo, and the amount of displacement at that time is about 1 μm. The maximum displacement of a piezo with a thickness of 10 mm is obtained when a voltage of about 10 kV is applied and the displacement is about 10 μm. In this way, the single-layer piezo actuator requires a high voltage to obtain a large displacement. Therefore, the single-layer piezo actuator is used for driving at a resonance frequency where a large displacement can be obtained even when a low voltage is applied, and for an application where a large output can be obtained even when a low voltage is applied in an ultrasonic frequency band as commonly used. On the other hand, the multilayer piezo actuator can apply a high electric field even at a low voltage by reducing the thickness of the piezoelectric ceramics per layer. For example, in the case of one layer of 100 μm, in the case of a multilayer piezo actuator with a thickness of 1 mm with 10 layers, a displacement of 1 μm can be obtained at 100 V. Additionally, in the case of a 10 mm multilayer piezo actuator with 100 layers, at 100 V a displacement of 10 μm can be obtained. The multilayer piezo actuator can thus increase the displacement while maintaining a low voltage by increasing the number of layers and is used in a wide frequency range other than the resonance frequency.