A power factor improving circuit including: a boost reactor that inputs a
rectified voltage obtained by rectifying an AC power supply voltage of an
AC power supply by a rectifier circuit; a main switch that inputs the
rectified voltage through the boost reactor and is turned on/off; a
converting section that converts a voltage obtained when the main switch
is turned on/off into a DC output voltage; and a control section that
controls turn-on/off of the main switch to shape an AC power supply
current to a sine wave form, controls an output voltage of the converting
section to a predetermined voltage, and controls a switching frequency of
the main switch according to a value of current flowing into the AC power
supply, or that of current flowing into the rectifier, or that of current
flowing into the main switch.