PURPOSE: To operate an oscillator stably against temperature change, fluctuation of the power source voltage, etc., by supplying a bias for determining a pulse ratio from the same bias resistance group.

CONSTITUTION: When a capacitor 32 is charged through a resistance 30 and a variable resistance 31 by a power source voltage Vcc 34, the base potential of a transistor TR 14 rises. When the base potential of the TR 14 becomes equal to a base potential Vh of a TR 15, the TR 14 is turned on, and TRs 17 and 22 having bases connected to the collector of the TR 14 and TRs 12 and 21 having bases connected to the collector of the TR 22 are turned on. The collector potential of the TR 21 becomes 0 approximately, and a TR 15 is turned off. Since the TR 12 is turned on, electric charge stored in the capacitor 32 is discharged through a resistance 13 and the TR 12, and the terminal voltage of the capacitor 32 is reduced. When the base potential of the TR 14 becomes equal to a base potential Vl of the TR 15, the TR 14 is turned off, and TRs 17, 22, 12, and 21 are turned off also, and discharge of the capacitor 32 is stopped to restart charging.