A power source apparatus has a power factor improving circuit which uses switching elements (4) as buildup :ransistors. According to the power source apparatus of the present invention, a current carrying ratio of the switching elements (4) is decided to be a value which is obtained by subtracting a number of proportional value of the power source current waveform from the maximum current carrying ratio. Since the apparatus according :o the present invention does not necessitate a detecting circuit of the power source voltage waveform for outputting an instruction waveform, the circuit (3,5,7) thereof is simple and is not affected by the voltage change of the power source and noise generated at the power source.