A power supply unit includes a first power supply configured to supply electric power to a load using electric power supplied from an external power source; a second power supply configured to store the electric power supplied from the external power source and supply the stored electric power to the load; a no-power detecting unit configured to monitor a voltage of the external power source and detect no electric power from the external power source; and an output switching unit configured to switch the electric power to be supplied to the load to the electric power supplied from the second power supply when the no-power detecting unit detects no electric power. A first setting value that is a target output voltage of the first power supply is larger than a second setting value that is a target output voltage of the second power supply.