PURPOSE: To limit stably an output current at overload, by detecting an overload current and turning off intermittently an ON-time of an oscillator by a thyristor.

CONSTITUTION: An output current of a blocking oscillator is detected by a transformer 18, rectified and smoothed by a diode 19 and a capacitor 20 and a voltage resulting from the current divided by resistors 21 and 23 controls the thyristor 17. Further, since a capacitor 14 is charged negatively with a value proportional to a voltage produced at a load terminal with a winding 25 and a diode 6, when a load current exceeds a value set by the resistors 21, 23 the thyristor 17 is turned on, a base potential of a transistor (TR)2 of the oscillator is a charging potential (negative) of the capacitor 14 so as to turn off the TR2. As a result, the maximum value of the load current is limited stably and the maximum value is determined optionally by a semifixed resistor.