The power supply unit has a sample-and-hold circuit that samples a voltage supplied from a power source to a load according to a sample timing signal and holds the voltage as an output set value for the DC/DC converter. The unit supplies a load with an output from the DC/DC converter controlled so that an output voltage of a power source becomes an output set value of the DC/DC converter when the voltage of the power source drops. Since a voltage corresponding to the voltage of the power source before the voltage drop is set to the output set value of the DC/DC converter, a stable power supply unit is obtained in which the difference is always small between the voltage of the power source in normal times and the output voltage from the DC/DC converter when the voltage of the power source temporarily fluctuates.