Abstract: A thickness-shear mode quartz oscillator characterized in that an addition mass fitted to a driving electrode of the oscillator for the purpose of adjusting its oscillation frequency has a non-circular shape or the fitting position thereof is deviated from the center of a quartz slice, or both in combination. Such addition mass provides different piezoelectric effects and effects of addition of the mass between the fundamental main vibration and harmonic vibrations and eliminates or restricts a "frequency abnormal phenomenon". Consequently, even when driven by a C-MOS type integrated circuit not having a frequency selection circuit in its oscillation loop, the thickness-shear mode quartz oscillator exhibits stable frequency-temperature characteristics.