A protection relay, comprising: an input circuit adapted to detect a state of an external device according to whether or not an external input voltage is larger than a preset threshold voltage; and an operation unit adapted to fetch an operation signal detected by the input circuit and to perform a protection relay operation, wherein the input circuit includes: switching means that is made conductive by a divided voltage obtained by voltage-dividing resistors that divide the external input voltage when the external input voltage is higher than or equal to the threshold voltage, and a photocoupler connected to the switching means, the photocoupler being operated by a constant current of a constant current output circuit supplying a constant current and being adapted to output the operation signal to the operation unit when the switching means is made conductive, characterised in that the protection relay further comprises: threshold voltage adjusting means comprising a plurality of resistors (Rf1 to Rfn) and a plurality of bypass switching elements, each of the plurality of bypass switching elements using another photocoupler, the threshold voltage adjusting means being adapted to adjust a voltage division ratio of the voltage-dividing resistors to adjust the threshold voltage for the external input voltage while maintaining the divided voltage obtained by the voltage-dividing resistors at a voltage that makes the switching means conductive. an input circuit adapted to detect a state of an external device according to whether or not an external input voltage is larger than a preset threshold voltage; and an operation unit adapted to fetch an operation signal detected by the input circuit and to perform a protection relay operation, wherein the input circuit includes: switching means that is made conductive by a divided voltage obtained by voltage-dividing resistors that divide the external input voltage when the external input voltage is higher than or equal to the threshold voltage, and a photocoupler connected to the switching means, the photocoupler being operated by a constant current of a constant current output circuit supplying a constant current and being adapted to output the operation signal to the operation unit when the switching means is made conductive, characterised in that the protection relay further comprises: threshold voltage adjusting means comprising a plurality of resistors (Rf1 to Rfn) and a plurality of bypass switching elements, each of the plurality of bypass switching elements using another photocoupler, the threshold voltage adjusting means being adapted to adjust a voltage division ratio of the voltage-dividing resistors to adjust the threshold voltage for the external input voltage while maintaining the divided voltage obtained by the voltage-dividing resistors at a voltage that makes the switching means conductive. switching means that is made conductive by a divided voltage obtained by voltage-dividing resistors that divide the external input voltage when the external input voltage is higher than or equal to the threshold voltage, and a photocoupler connected to the switching means, the photocoupler being operated by a constant current of a constant current output circuit supplying a constant current and being adapted to output the operation signal to the operation unit when the switching means is made conductive, characterised in that the protection relay further comprises: threshold voltage adjusting means comprising a plurality of resistors (Rf1 to Rfn) and a plurality of bypass switching elements, each of the plurality of bypass switching elements using another photocoupler, the threshold voltage adjusting means being adapted to adjust a voltage division ratio of the voltage-dividing resistors to adjust the threshold voltage for the external input voltage while maintaining the divided voltage obtained by the voltage-dividing resistors at a voltage that makes the switching means conductive. threshold voltage adjusting means comprising a plurality of resistors (Rf1 to Rfn) and a plurality of bypass switching elements, each of the plurality of bypass switching elements using another photocoupler, the threshold voltage adjusting means being adapted to adjust a voltage division ratio of the voltage-dividing resistors to adjust the threshold voltage for the external input voltage while maintaining the divided voltage obtained by the voltage-dividing resistors at a voltage that makes the switching means conductive.