PURPOSE: To simplify supervisory operation by adding gate circuits and gate driver circuits to a level supervisory output and then by mutually connecting respective gate circuit outputs and leading them to a level supervisory meter.

CONSTITUTION: Gate circuits 2a, 2b...2n are provided corresponding to level supervisory items and gate driver circuits 3a...3n are opened corresponding to respective decision signals of circuits 2a...2n. Those decision signals are supplied from gate synchronous circyit 7 to circuits 3a...3n at intervals of fixed time and supervisory level outpus 1a...1n are connected to all of circuits 2a...2n to exercise a level supervision. Consequently, supervisory operation can be simplified even in case of the supervision over several levels.