To cancel sensor detection setting at the installation place of a sensor.

A current circuit 205 from a sensor detection circuit 21 to a terminating device 204 and a reference potential 25 for detecting the state of an OFF type sensor 203 to be in a closed state at the time of monitoring and an open state at the time of abnormality detection is provided, the OFF type sensor is serially connected to the current circuit and a sensor detection cancellation setting switch 26 is parallelly connected between the sensor detection circuit and the reference potential. By changing the size of a current flowing out to the current circuit from the sensor detection circuit at the time of monitoring, at the time of the abnormality detection and at the time of monitoring cancellation setting, the sensor detection cancellation setting switch is installed on the outside of an alarming device. Thus, it is optionally provided even near the sensor installed in a place away from the alarming device and the monitoring set sensor is turned to monitoring cancellation.