PURPOSE: To obtain a sampling synchronization monitor circuit in which the revision of a synchronization discrimination specification is facilitated with simple configuration.

CONSTITUTION: A synchronization width generating circuit 1 generates an own station synchronization discrimination width signal for a predetermined width before and after rising of an own station sampling signal and a synchronization discrimination circuit 2 discriminates whether or not instantaneous synchronization takes place depending whether an external sampling signal rises or not within a width of the own station synchronization discrimination width signal. The discrimination signal is inputted to a microcomputer via the synchronization discrimination hold circuit 3 and an interface circuit 7. The microcomputer discriminates a synchronization state or an asynchronization state depending on momentary synchronization or momentary asynchronization consecutive for a predetermined period.