A current source circuit is contained in an output control IC and amplifies a feedback current produced by a photocoupler by 7 to 10 times. The current amplifier has an adequate current amplification factor and is composed of an IC. A capacitor connected to the oscillator is charged by a current amplified at the current amplifier and a period of the output of the oscillator is controlled by the charging current. A conductive period of the switching device connected to the oscillator through a driver is also controlled. Accordingly, the amount of energy supplied to the secondary side of the switching transformer is also controlled and a stable secondary dc output voltage is produced. A stable voltage control is obtained over a wide range of loads, from a light load to a heavy load. Because the current amplification factor is stable against an environmental temperature condition and has little variation, there is no abnormal oscillation of the power source circuit due to a ripple component included in the secondary dc output voltage and a stable controlled dc voltage stabilizer can be realized.