A network system including: a first sensor device which includes a first sensor unit which detects an action of a detection object; and a first transmission unit which transmits first data regarding the action and first identification information to a server via a first base station: a second sensor device which includes; a second sensor unit which obtains sensing data; and a second transmission unit which transmits second data including the sensing data and second identification information to the server via a second base station; wherein the server is configured to: associate the first and second data based on the detection of the action; to access predetermined normal data correlating data; and, detect an abnormal condition if a combination of the action detected via the first sensor unit and the sensing data obtained by the second sensor unit conflict with the correlated normal data.