PURPOSE: To attain the reduction of power source consumption while supplying a stable voltage in spite of a load fluctuation without necessitating an auxiliary power source circuit including a power source transformer.

CONSTITUTION: The power source circuit for a CRT display device in a CRT display device 1 displaying visibly picture information is provided with a power source transformer 3 at primary side a power is supplied from an AC. power source 2, a driving circuit 4 provided at the secondary side of the power source transformer 3 and whose power is supplied from the transformer 3. a switching circuit 5 allowing a power supplied from the transformer 3 to the driving circuit 4 to be turned ON and OFF, a switching control circuit 6 performing the ON-OFF controls of the switching circuit 5 and a pulse width power supply control circuit 7 controlling the power amount of an AC. power source supplied to the primary side of the transformer 3 with pulse widths and stabilizing the output voltage of the secondary side of the transformer 3 by making the supplying power amount larger while widening pulse widths when the output voltage of the secondary side of the transformer 3 is lower than a prescribed voltage and by making the supplying power amount smaller while narrowing pulse widths when the output voltage of the secondary side of the transformer 3 is higher than the prescribed value.