A power factor improving circuit for reducing a harmonic current component contained in an AC input current and for achieving an excellent power factor and efficiency in a circuit having a rectifier circuit, provided between a rectifier circuit and a half-bridge-type inverter, including a discharging diode and a smoothing capacitor connected in series to each other and in parallel between output terminals of the rectifier circuit, a charging half-bridge rectifier connected to the smoothing capacitor, and a high-frequency capacitor and high-frequency inductor connected in series to each other and between an input terminal of the charging half-bridge rectifier and a connecting point of two transistors of the half-bridge-type inverter.