A quartz crystal oscillator has an amplifier, a capacitor, a resistor and a quartz crystal unit comprised of a case, a quartz crystal tuning fork resonator vibratable in a flexural mode and a lid connected to the case. At least one groove with a first surface opposite a first side surface and a second surface connected to the first surface through a third surface is formed in each of first and second main surfaces of each of first and second quartz crystal tuning fork tines. The width of each groove is greater than a distance in the width direction of the groove measured from an outer edge of the groove to an outer edge of the corresponding tuning fork tine. A first electrode is disposed on the first surface of the groove and a second electrode is disposed on the first side surface so that the first electrode has an electrical polarity opposite to the electrical polarity of the second electrode. Each of an overall length of the resonator and a length of the groove is determined so that a series resistance R1 of a fundamental mode of vibration of the resonator is less than a series resistance R2 of a second overtone mode of vibration thereof. The tuning fork resonator is electrically connected to the amplifier, the capacitor and the resistor.