A method of manufacturing a quartz crystal unit comprises disposing a metal film on opposite surfaces of a quartz crystal wafer and then etching the wafer to form a two-line tuning fork resonator vibratable in a flexural mode of an inverse phase. A groove is formed in the opposite main surfaces of both tines, and the length of each groove is determined relative to the length of the resonator so that the series resistance of the fundamental mode of vibration of the resonator is less than the series resistance of the second overtone mode of vibration thereof. The resonator is then mounted in case after which the resonant frequency of the resonator is adjusted.