A power supply protective device comprising: a first switch provided between a first terminal (T11) to which at least one of a load and a charger is connected and a second terminal (T12) 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 (T11); 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 (T11) to which at least one of a load and a charger is connected and a second terminal (T12) 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 (T11); 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. <EOS>