To improve response speed and ripple removal rate, lengthen battery service life and reduce size and cost by varying the amplitude of sawtooth voltage according to fluctuation in input direct-current voltage.

An amplified differential voltage Vd between an output direct- current voltage Vout from a power converting section 1 and a reference voltage V0 from a reference voltage section 2, is input to a comparing section 5. Further, a sawtooth voltage S the amplitude of which corresponds to fluctuation in input direct-current voltage Vin, is input from an oscillating section 4 to the comparing section 5. Then the comparing section 5 compares the amplified differential voltage Vd with the sawtooth voltage S, and outputs a control signal C, generated by PWM modulation, to the power converting section 1, which then amplifies the output direct-current voltage Vout according to the control signal C.