In a power supply apparatus, a switching circuit selectively applies one of an input voltage of a power converting circuit and an output voltage of the power converting circuit to a voltage reducing power supply circuit. When the input voltage is applied to the power converting circuit, the switching circuit selects this input voltage, so that the output voltage of the voltage reducing power supply circuit can quickly rise. Thereafter, when the output voltage of the power converting circuit exceeds this output voltage, the switching circuit selects the output voltage of the power converting circuit. As a result, a difference between the input voltage and the output voltage of the voltage reducing power supply circuit is decreased. Thus, a power loss is lowered and noise produced in the power converting circuit is suppressed.