To provide the fail-safe system used for a measurement control unit having a level detecting device built in.

An electric circuit that the system is equipped with so as to impart an output signal according to a state recognized by a sensor is equipped generally with comparators IC1 and IC2, a phase detector IC3, and a triple- redundancy driving circuit 12 which drives a relay 14. The IC1 secures a display of an abnormal state in system output by an electrode fault which is possibly caused by excessive contamination in operation with water in a normal state. The comparator IC2 mainly discriminates between a water state and a steam state. The phase detector IC3 secures the absence of output in an abnormal state or when some fault state is present. Consequently, the output is generated only when the sensor is normal.