To provide a quartz-crystal oscillator, equipped with a flexural crystal resonator the output signal of which is low in a equivalent series resistance R1 at the frequency of fundamental wave mode vibration and which is high in quality factor (Q-value) and has a very small tuning fork like shape.

This quartz-crystal oscillator is equipped with the flexural quartz resonator having tuning fork arms and a tuning fork base part, grooves formed on the upper and lower surfaces of the tuning fork arms or tuning fork arms and base part, and electrodes disposed to the side face sections of the grooves. The equivalent series resistance R1 of the resonator at the frequency of fundamental wave mode vibration is smaller than that R2 at the frequency of second harmonic mode vibrations. In addition, the output signal of the resonator has a fundamental wave mode frequency, depending upon the relationship between the amplification factor of an amplifier circuit and the feedback rate of a feedback circuit.