A crystal resonator comprises an AT-cut crystal vibrating element that is driven by a thickness-shear mode and is in the shape of a rectangular plate. A pair of excitation electrodes is formed, facing front and rear surfaces of the crystal vibrating element. Each of the excitation electrodes is formed in the shape of a quadrangle as viewed from the top, and mass adjustment portions are formed at least two opposite sides of each of the excitation electrodes formed on the front and rear surfaces.