To obtain a small-sized power unit with an improved power factor, by interposing a power-factor improving reactor comprising both two coils with the same number if returns and a closed-magnetic-path core between the rectifier circuit and filter circuit of its input portion.

When an input voltage is 100V, a switch SW is turned on to constitute a voltage doubter rectifier circuit, so that a peak currant equal to an input current flows into one of the respective halved coils of a reactor L every half period. Therefore, the reactor L serves as two reactors of N/2 in number of turns. As a result the reactor L caries a current whose value is 1/2(1/2) times as larger as the RMS value of the input current flows in full period to make number of turns of the reactor L apparently N/2 and reduce its copper loss by half. On the the hand, when the input voltage is 200V, the switch SW is turned off to constitute a dull-wave rectifier circuit. In this case, a current equal to the input current flows in the reactor L to make the reactor L serve a reactor of N in the number of turns.