PURPOSE: To stabilize the oscillation and to attain circuit integration by providing a start transistor (TR) and applying switching control to a switching TR in response to a base-emitter voltage level of the oscillation TR so as to oscillate the oscillation TR.

CONSTITUTION: Since the base-emitter voltage of a TR Q5 is high when a TR Q3 is turned on, the emitter potential of a TR Q7 is high and the TR Q7 is turned off. 0n the other hand, a current Il flowing to a coil L is larger than a collector current IC5 of the TR Q5 and the TR Q3 is turned off, then the base-emitter voltage of the TR Q5 is lowered. The TR Q7 is turned on to draw the base current of the TR Q3 and the TR Q3 is controlled in the on-state. Similarly, the oscillation is continued. Thus, no low resistance is required and the circuitry is suitable for circuit integration and the oscillation is stabilized by the action of the start TR Q7.