A quartz crystal unit has a quartz crystal tuning fork resonator that is capable of vibrating in a flexural mode and that has a quartz crystal tuning fork base, and first and second quartz crystal tuning fork tines. The quartz crystal tuning fork resonator is mounted on a mounting portion of the case after which the oscillation frequency of the quartz crystal tuning fork resonator is adjusted. The quartz crystal unit comprising the quartz crystal tuning fork resonator has a series resistance R.sub.1 of a fundamental mode of vibration less than a series resistance R.sub.2 of a second overtone mode of vibration. At least one groove is formed in at least one of opposite main surfaces of each of the first and second quartz crystal tuning fork tines so that a width of the at least one groove is greater than a distance in the width direction of the at least one groove measured from an outer edge of the at least one groove to an outer edge of the corresponding one of the first and second quartz crystal tuning fork tines.