A power supply control apparatus monitors a high voltage, an intermediate voltage and a low voltage and detects voltage variation amounts of those voltages. When an absolute value of the voltage variation amount is greater than a predetermined variation amount, the power supply control apparatus corrects the voltage of large variation by controlling a DC-DC converter based on the voltage variation amount. The power supply control apparatus further estimates voltage variation amounts of the voltages other than the voltage of large variation based on the voltage variation amount of large variation and corrects the voltage of estimated large variation by controlling the DC-DC converter based on the estimated voltage variation amount.