A zero traffic state that is a non-communication state derived from any fault is discriminated from a no-operation state derived from, power discontinuity, and a communication device that is in the zero traffic state is efficiently sensed in consideration of a zero traffic period that is normally observed in a certain installation place or use environment. A network monitoring device cyclically notifies a maintenance person of a communication device, for which a time difference between a final access time and a finally passed traffic generation time exceeds a threshold calculated by multiplying a previously observed maximum, value of the time difference between the final access time and the finally passed traffic generation time by a coefficient that is a safety factory as a device that is In a zero traffic suspected state.