PURPOSE: To prevent generation of abnormal sounds when oscillation is started, by feeding the On power to a switching element from a power supply in conformity to signal given by a signal output part.

CONSTITUTION: When power is put on, a voltage full-wave rectified by a full- wave rectifier 1 is impressed on the collector of a switching element 5. At that time, the terminal voltage VC of a capacitor 20 becomes higher than this collector voltage VT, and a diode 15 remains off, so that the capacitor 20 is charged via a resistance 17. The terminal voltage VC of the capacitor 20 rises accordingly to cause by and by a transistor 21 to be turned on, and thereby a transistor 24 is also turned on, when the On voltage will be impressed on the base of the switching element 5 to cause it to be turned on. As a result, self-excited oscillation is started. This allows continued oscillation even though the oscillation is started at a lower supply voltage, and generation of abnormal sounds at the starting time of oscillation is prevented.