PURPOSE: To make a circuit small-sized and simplified and to obtain a long life of the circuit by controlling timing for generating switching pulses so as to output a constant voltage from an inverter circuit.

CONSTITUTION: A transistor(Tr) driving circuit 8a is used to turn on and off Tr3, Tr4 at the same time and to control an operation of an inverter circuit 4. When Tr3 and Tr4 are both turned on, no current flows in a primary winding of a circuit 4 so that the circuit 4 is turned off. When Tr3 and Tr4 are both turned off, a current flowing in a resistance r0 is made to flow to bases of Tr1 and Tr2. When Tr1, for example, is first made to be on an active state, Tr1 is turned on through a winding L1' and a current of a primary winding L1 corresponding to an excitation current and a load current in the circuit 4 is made to flow in Tr1, so that an inductance of a transformer composing the circuit 4 rapidly decreases. When a voltage between a collector and an emitter in Tr1 is increased to decrease a voltage across L1, Tr1 is turned off and the voltage across L1 is inverted to repeat similar operations in Tr2, so that an output voltage of the circuit 4 is controlled to be constant and it is supplied to a lamp 7.