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 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, when obtaining a voltage from the voltage detection unit as an open voltage in a case where the second switch is brought into the closed state and the first switch is brought into the open state during discharging of the power supply, and obtaining a voltage from the voltage detection unit as a close voltage in a case where the second switch is brought into the closed state and the first switch is brought into the closed state during discharging of the power supply, is configured to diagnose the first switch as having no failure when a first condition is satisfied that a difference voltage between the close voltage and the open voltage is not less than a third threshold voltage corresponding to the reference voltage, and to diagnose the first switch as having either an open failure or a closed failure when the first condition is not satisfied. 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 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, when obtaining a voltage from the voltage detection unit as an open voltage in a case where the second switch is brought into the closed state and the first switch is brought into the open state during discharging of the power supply, and obtaining a voltage from the voltage detection unit as a close voltage in a case where the second switch is brought into the closed state and the first switch is brought into the closed state during discharging of the power supply, is configured to diagnose the first switch as having no failure when a first condition is satisfied that a difference voltage between the close voltage and the open voltage is not less than a third threshold voltage corresponding to the reference voltage, and to diagnose the first switch as having either an open failure or a closed failure when the first condition is not satisfied.