PURPOSE: To prevent an unsatisfied influence from spreading to the system total unit, by means of ceasing or reducing a transmission of the signal, with such case detected that the synch current continues flowing as a supervisory signal when caused an anomalousness in each controlled unit.

CONSTITUTION: At a normal time though a supervisory pulse is normally carried out from an output terminal X, said pulse is according to a voltage across a condenser Cn charged and discharged by the charge and discharge circuit determined by a resistor r2, the condenser Cn and a resistor r3, and a resistance across base-emitter of a transistor Tn. An extremely short length of sending time of the supervisory pulse, as compared with that required for the total length of time, causes the voltage to rise in very few level, with the transistor Tn kept in a state of off and a transistor T1 driven by the normal pulse. When an anomalousness is caused in a logical circuit M, the voltage acriss the condenser Cn rises to turn on the transistor Tn with the transistor T1 keeping a state of off.