Capacitor elements are connected in series and charged until a voltage between terminals of a capacitor element whose capacitance is lowest among the capacitor elements reaches a withstanding voltage. When the voltage between the terminals of the capacitor element reaches the withstanding voltage, the capacitor elements are connected in parallel, and electric charge of the capacitor element is transferred to the other capacitor elements. Hereafter, an operation of switching between series connection and parallel connection is repeated such that the capacitor elements are connected in series and charged again when the voltage between the terminals of the capacitor element is reduced to a full charge voltage due to an outflow of the electric charge, and the capacitor elements are connected in parallel when the voltage between the terminals of the capacitor element reaches the withstanding voltage.