A power supply device having a first load operating with a first voltage in a normal operation mode, a second load operating with a second voltage lower than the first voltage in the normal operation mode, and a third load operating with the second voltage in the normal operation mode and a power-saving mode. An auxiliary second-voltage generating circuit configured to, when a main supply voltage is supplied from a main power supply circuit and a primary second-voltage generating circuit does not generate the second voltage with a third-load electric path for carrying therethrough the main supply voltage from the main power supply circuit to the third load without passing through the primary second-voltage generating circuit being broken by a switching circuit, generate the second voltage from the main supply.