A power supply protective device comprising: a first switch provided between a first terminal to which at least one of a load and a charger is connected and a second terminal to which a power supply is connected; a switching circuit connected in parallel to the first switch and including a second switch and a voltage drop element connected in series to the second switch, the voltage drop element causing a voltage drop of a reference voltage by a current flow; a voltage detection unit which is configured to detect a first voltage as a voltage of the first terminal and a second voltage as a voltage of the second terminal, a control unit which is configured to give, to the first switch and the second switch, an open instruction to bring the respective switch into an open state and a close instruction to bring the respective switch into a closed state to control switching, and which is configured to obtain a voltage from the voltage detection unit, characterized in that the control unit is further configured to obtain the first voltage and the second voltage from the voltage detection unit in a case where the second switch is controlled to be in the closed state and the first switch is controlled to be in the open state during discharging of the power supply, to obtain an open voltage by subtracting the first terminal voltage from the second terminal voltage, to obtain the first voltage and the second voltage from the voltage detection unit in a case where the second switch is controlled to be in the closed state and the first switch is controlled to be in the closed state during discharging of the power supply, to obtain a close voltage by subtracting the first terminal voltage from the second terminal voltage, and to diagnose the first switch as having a failure or not on the basis of a comparison result obtained by comparing the close voltage and the open voltage, respectively, with a fourth threshold voltage corresponding to the reference voltage, wherein the control unit is configured to diagnose the first switch as having a closed failure when the open voltage is less than the fourth threshold voltage, and to diagnose the first switch as having an open failure when the close voltage is not less than the fourth threshold voltage. a first switch provided between a first terminal to which at least one of a load and a charger is connected and a second terminal to which a power supply is connected; a switching circuit connected in parallel to the first switch and including a second switch and a voltage drop element connected in series to the second switch, the voltage drop element causing a voltage drop of a reference voltage by a current flow; a voltage detection unit which is configured to detect a first voltage as a voltage of the first terminal and a second voltage as a voltage of the second terminal, a control unit which is configured to give, to the first switch and the second switch, an open instruction to bring the respective switch into an open state and a close instruction to bring the respective switch into a closed state to control switching, and which is configured to obtain a voltage from the voltage detection unit, characterized in that the control unit is further configured to obtain the first voltage and the second voltage from the voltage detection unit in a case where the second switch is controlled to be in the closed state and the first switch is controlled to be in the open state during discharging of the power supply, to obtain an open voltage by subtracting the first terminal voltage from the second terminal voltage, to obtain the first voltage and the second voltage from the voltage detection unit in a case where the second switch is controlled to be in the closed state and the first switch is controlled to be in the closed state during discharging of the power supply, to obtain a close voltage by subtracting the first terminal voltage from the second terminal voltage, and to diagnose the first switch as having a failure or not on the basis of a comparison result obtained by comparing the close voltage and the open voltage, respectively, with a fourth threshold voltage corresponding to the reference voltage, wherein the control unit is configured to diagnose the first switch as having a closed failure when the open voltage is less than the fourth threshold voltage, and to diagnose the first switch as having an open failure when the close voltage is not less than the fourth threshold voltage.