A network system is provided in which a plurality of information processing devices are mutually connected through a network.A monitored device included in a plurality of the information processing devices comprises: a selecting device configured to select a preset number of identification information pieces; a monitored device sending device configured to send a monitor request message; and a response message sending device configured to send a response message indicative of the operating state.An information processing device which is a final destination of the respective sent monitor request messages is a monitoring device, which comprises: a reception device configured to receive the monitor request message; a monitor message sending device configured to send the monitored device the monitor messages; and a judgment device configured to judge that abnormality has occurred in the operating state.