PURPOSE: To form a crystal oscillator having a very small frequency change due to temperature change, by combining a crystal oscillator of 5°, -5° cut and a crystal oscillator of 20°, -20° cut.

CONSTITUTION: The 5°, -5° cut crystal oscillator is used for one of two oscillating circuits, and the 20°, -20° cut crystal oscillating circuit is used for the other. A beat frequency is extracted from an oscillating signal at a beat detecting circuit 3, frequency-divided up to a suitable frequency at a frequency-dividing stage 4 for use as a clock. It is confirmed that the frequency versus temperature characteristic of the crystal oscillator formed in this way is within ±2ppm without the temperature compensation at a temperature range of -60°C∼+85°C. Thus, the crystal oscillator with very small frequency change due to temperature change is formed.