An apparatus for monitoring automatically plural sheets of printed papers, includes a plurality of photosensors placed across the printed papers' moving direction for scanning and detecting a printed surface of each printed paper to produce analog signals designating dark levels of the printed surface thereof when each of the printed papers is being transferred, an AD converter for converting the analog signals into digital signals at a plurality of sampling points, a standard memory for storing such digital signals, a plurality of monitoring memories for storing such digital signals, circuitry for comparing the digital signals of the standard memory with the digital signals of the monitoring memories at the corresponding sampling points to decide whether or not the digital signals of the monitoring memories are within a tolerance range of the digital signals of the standard memory so that either "NO" signals or "YES" signals are produced, circuitry for counting only such "NO" signals to produce an "irregular" or "non-identity" signal when the number of such "NO" signals sums to a predetermined value, and circuitry for electrically compensating shifting of the papers when the digital signals of the standard memory are compared with the digital signals of the monitoring memories.