According to one embodiment, there is provided a protection relay. The protection relay includes an input circuit that detects a state of an external device according to whether or not an external input voltage is larger than a preset threshold voltage. The input circuit includes switching unit 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 that is operated by a constant current of a constant current output circuit supplying a constant current and outputs an operation signal to the operation unit when the switching unit is made conductive.