A piezoeletric resonator includes: a substrate (105); a lower electrode (103) formed on or above the substrate; a piezoeletric body (101) formed on or above the lower electrode; an upper electrode (102) formed on or above the piezoeletric body; and a cavity (104) under a vibrating portion formed by the lower electrode, the piezoeletric body, and the upper electrode. Where a resonant frequency of vibration with a thickness of the vibrating portion being a half of a wavelength is taken as fr, an average of ultrasonic velocity in a material forming the cavity is taken as Vc, and a value determined based on the resonant frequency fr and the average of ultrasonic velocity Vc is taken as λc (=Vc/fr), a depth of the cavity is set so as to be equal to or larger than n×λc/2-λc/8 and equal to or smaller than n×λc/2+λc/8.