PURPOSE: To keep the frequency of an oscillator to nearly a constant value by providing two inverting circuit means, an adder means of their outputs, a feedback means and a control means controlling the oscillating period of an oscillated output signal, and adjusting the frequency with the control means in response to a temperature change.

CONSTITUTION: An oscillator 10 includes 1st and 2nd differential circuit stages 12, 14 and a signal passing through a 2nd circuit stage is retarded more than a signal through a 1st circuit stage 12 because a propagation delay time is fed by delay elements T1, T2. The circuit stages 12, 14 invert a differential signal fed to each input terminal and the oscillator 10 oscillates through the repetition of the inversion. Output signals from the circuit stages 12, 14 are added at connecting points 16, 18 resulting in forming an oscillated signal. Then the oscillated signal is fed back via lead wires 20, 22 and passive delay elements T3, T4. Thus, the gain of the two inverting circuit means is controlled relatively in response to the temperature change in the oscillator, then the oscillating period is kept constant independently of the temperature change.