To provide an optical abnormal temperature monitoring sensor that does not need a power source and can remotely monitor that temperature deviates from a normal range.

An optical path 23 between an optical fiber 21 and a mirror 23 can be cut off or connected, when the shape of a metallic body 24 is changed by the change of the ambient temperature of the optical abnormal temperature monitoring sensor 20. Also, the ambient temperature of the sensor 20, that deviates from the normal range can be remotely monitored by pre-setting temperature that the path 23 is switched from a cut-off state to a connection state or from the connection state to the cut-off state.