Charging time of a capacitor is shortened while avoiding erroneous firing due to distortion of a power supply voltage and sustaining an inrush current at a constant level. In the charge control method of a capacitor in a thyristor converter comprising a thyristor for rectifying an AC voltage, a CPU delivering an on/off control signal to a thyristor driver for driving the thyristor, a voltage detection sensor for measuring the AC voltage and the capacitor charging voltage, and the capacitor connected with a DC circuit, the AC voltage and the capacitor charging voltage are measured by means of the voltage detection sensor, differential voltage between the firing phase voltage of the thyristor and the capacitor charging voltage is determined, and the thyristor is fired only when the firing phase voltage of the thyristor becomes lower than a specified voltage determined from the differential voltage.