PURPOSE: To fill up the self monitoring function by arranging the constitution such that two unit relays, which constitute a digital protective relay device, monitor each other's operation.

CONSTITUTION: For a digital protective relay device 18, in the case that unit relay parts 16 and 17 are normal, both normal signals 13a and 17c are outputted from these relay parts in synchronizm with the respective clock pulses in the relay parts 16 and 17 with periods of ι1 and ι2, and the output state of the normal signals outputted from one unit part is monitored with the other relay part. Accordingly, it follows that, as a relay device 18, self monitoring is performed covering the whole range of the device 18 including the clock pulse generators in the unit relays 16 and 17, and a highly reliable digital protective relay device can be gotten.