A power supply unit comprising: a power source; a bidirectional DC/DC converter with a first terminal and a second terminal, the first terminal being connected to the power source; a sample-and-hold circuit sampling and holding a voltage corresponding to a voltage supplied from the power source to a load according to a sample timing signal; and a control circuit that controls the bidirectional DC/DC converter, wherein an output voltage output from the bidirectional DC/DC converter to the load is changed according to a voltage fluctuation of the power source, and an output set value of the bidirectional DC/DC converter is controlled by the held voltage characterized by: an auxiliary power supply connected to the second terminal of the bidirectional DC/DC converter, wherein the bidirectional DC/DC converter obtains an output voltage from the auxiliary power supply, and wherein, when a voltage of the power source drops, the control circuit is configured to: increase the voltage from the auxiliary power supply, and supply the increased voltage to the load; and control the output voltage output from the bidirectional DC/DC converter to the load such that a voltage proportional to the output voltage is the output set value. a power source; a bidirectional DC/DC converter with a first terminal and a second terminal, the first terminal being connected to the power source; a sample-and-hold circuit sampling and holding a voltage corresponding to a voltage supplied from the power source to a load according to a sample timing signal; and a control circuit that controls the bidirectional DC/DC converter, wherein an output voltage output from the bidirectional DC/DC converter to the load is changed according to a voltage fluctuation of the power source, and an output set value of the bidirectional DC/DC converter is controlled by the held voltage characterized by: an auxiliary power supply connected to the second terminal of the bidirectional DC/DC converter, wherein the bidirectional DC/DC converter obtains an output voltage from the auxiliary power supply, and wherein, when a voltage of the power source drops, the control circuit is configured to: increase the voltage from the auxiliary power supply, and supply the increased voltage to the load; and control the output voltage output from the bidirectional DC/DC converter to the load such that a voltage proportional to the output voltage is the output set value. an auxiliary power supply connected to the second terminal of the bidirectional DC/DC converter, wherein the bidirectional DC/DC converter obtains an output voltage from the auxiliary power supply, and wherein, when a voltage of the power source drops, the control circuit is configured to: increase the voltage from the auxiliary power supply, and supply the increased voltage to the load; and control the output voltage output from the bidirectional DC/DC converter to the load such that a voltage proportional to the output voltage is the output set value. increase the voltage from the auxiliary power supply, and supply the increased voltage to the load; and control the output voltage output from the bidirectional DC/DC converter to the load such that a voltage proportional to the output voltage is the output set value.