To improve the properties of a piezoelectric thin film device by effectively suppressing sub-resonance of a piezoelectric thin film resonator.
In the piezoelectric thin film resonator 1, the planar shape of a free vibration region 192 separating from a support substrate 11 a piezoelectric thin film 15 on which a lower surface electrode 14 and an upper surface electrode 16 are formed in an opposite region 191 is a pentagon, and five sides 135 to 139 constituting the contour 134 of the pentagon are respectively un-parallel to one another. It is because of consideration that acoustic impedance of the piezoelectric thin film 15 greatly differs between the inside and the outside of the free vibration region 192 to make a part being the contour 134 closer to a fixed end. Namely, since elastic waves leaked from the opposite region 191 included by the free vibration region 192 are reflected by the part being the contour 134, the reflected elastic waves cause standing waves to prevent from being a cause of sub-resonance.
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