The present invention provides a power supply protection device, a power supply device, and a switch fault diagnosis method. (1) If a power supply protection device acquires an open voltage for a case where a second switch is closed and a first switch is open when a power supply is discharging and acquires a closed voltage for a case where the second switch is closed and the first switch is closed when the power supply is discharging, the power supply protection device determines whether the first switch has a fault on the basis of the open voltage and closed voltage, and (2) when a first command process has been executed in which a close command is issued for the first switch and second switch, if the voltage difference between a first voltage and second voltage detected by a voltage detection unit during the execution of the first command processing is less than a first threshold voltage, a determination is made that the first switch does not have an open fault, and/or if a third voltage detected by the voltage detection unit during the execution of the first command processing is greater than or equal to a second threshold voltage, a determination is made that the second switch does not have an open fault.