The power factor improving circuit has a current detecting resistor Rsh for detecting current conducting through a set up reactor L1 or current conducting through a switch Q1, an output voltage detector 11 which amplifies a difference between output voltage Eo and reference voltage Vref to generate error voltage, a variable gain amplifier 15 which amplifies voltage which is proportional to current detected by the current detecting resistor Rsh by varying gain in accordance with a value of the error voltage of the output voltage detector 11, and a pulse width modulator 14 which generates a pulse signal whose pulse width is controlled in accordance with a value of output of the variable gain amplifier 15 and which applies the pulse signal to the switch Q1 to control the output voltage Eo to predetermined voltage.