PURPOSE: To provide the subject oscillator which can work with a power supply of low voltage like 3V or so and can secure the oscillation output of high purity.

CONSTITUTION: A time constant circuit 14 contains a capacitor 14b which is connected in series to a variable impedance 13 that varies in response to the temperature detected by a temperature sensor 12 connected thermally to a crystal resonator 11a of a crystal oscillator 11. The charging voltage of the capacitor 14b is applied to the input of a 1st or 2nd comparator 15 or 16, and at the same time the different levels of voltage divided by a resistance voltage dividing circuit 17 ere applied to each other input of both comparison. Then a flip-flop 18 is set and reset with the output of comparison. The output of the flip-flop 18 is applied to a counter 20 which counts the oscillation frequency of the oscillator 11 as a gate signal. The count value of the counter 20 is given to a temperature compensating circuit 21 which stores the compensation data to compensate the change of the oscillation frequency against the temperature change of the oscillator 11. Then the temperature compensation is applied to the oscillation frequency based on the compensation data corresponding to the count value of the counter 20.