A power supply device, comprising: first and second electric power supply units (10A, 10B) that supply electric power to a load unit (2A, 2B); a switch (Q1, Q2) that conducts or interrupts conduction current that flows through a bi-directional circuit to which electric power is supplied from the first electric power supply unit and in which current is capable of flowing in a positive direction, and to which electric power is supplied from the second electric power supply unit and in which current is capable of flowing in a negative direction that is a direction opposite to the positive direction; a current sensor that outputs a detection voltage according to the conduction current; and a controller that controls the switch (Q1, Q2) by calculating the conduction current based on the detection voltage output from the current sensor, wherein the controller controls the switch (Q1, Q2) according to the conduction current calculated based on an absolute value of a difference between a reference voltage (Vth) that is the detection voltage output during a non-conductive state in which current is not flowing through the bi-directional circuit, and the detection voltage output during a conductive state in which current is flowing through the bi-directional circuit, the controller turns OFF the switch (Q1, Q2) based on a heat quantity of the bi-directional circuit defined according to the conduction current and a predetermined positive direction interruption threshold, when the conduction current is flowing in the positive direction, and the controller turns OFF the switch (Q1, Q2) based on the heat quantity of the bi-directional circuit defined according to the conduction current and a predetermined negative direction interruption threshold, when the conduction current is flowing in the negative direction. first and second electric power supply units (10A, 10B) that supply electric power to a load unit (2A, 2B); a switch (Q1, Q2) that conducts or interrupts conduction current that flows through a bi-directional circuit to which electric power is supplied from the first electric power supply unit and in which current is capable of flowing in a positive direction, and to which electric power is supplied from the second electric power supply unit and in which current is capable of flowing in a negative direction that is a direction opposite to the positive direction; a current sensor that outputs a detection voltage according to the conduction current; and a controller that controls the switch (Q1, Q2) by calculating the conduction current based on the detection voltage output from the current sensor, wherein the controller controls the switch (Q1, Q2) according to the conduction current calculated based on an absolute value of a difference between a reference voltage (Vth) that is the detection voltage output during a non-conductive state in which current is not flowing through the bi-directional circuit, and the detection voltage output during a conductive state in which current is flowing through the bi-directional circuit, the controller turns OFF the switch (Q1, Q2) based on a heat quantity of the bi-directional circuit defined according to the conduction current and a predetermined positive direction interruption threshold, when the conduction current is flowing in the positive direction, and the controller turns OFF the switch (Q1, Q2) based on the heat quantity of the bi-directional circuit defined according to the conduction current and a predetermined negative direction interruption threshold, when the conduction current is flowing in the negative direction.