To decrease the number of detection switches and to reduce the facility cost and construction cost while maintaining the reliability of operations by stopping main equipment when at least two switches among a signal monitor switch and two detection switches decide that the main equipment should be stopped.

The signal monitor switch 24 and detection switches 11 and 12 decide whether or not the main equipment 2 should be stopped according to the analog signal 21a outputted by a transmitter 21 and detected numerals and the main equipment 2 is not stopped when there is no abnormality. Here, even if one of the signal monitor 24 and detection switches 11 and 12 malfunctions to decide abnormality although the actually detected numerals are not abnormal, a command of stopping the main equipment 2 is not outputted from a 2-out-of-3-line circuit 13 as long as the remaining two switches are operating normally. Consequently, the main equipment is never stopped owing to the malfunction of one switch.