A network system including a plurality of mobile devices is provided. The plurality of mobile devices each have a sensor function and a wireless communication function, the mobile devices communicating sensor information about a target with one another and tracking the target; a target searching section configured to obtain sensor information while the plurality of the mobile devices move and for reporting the sensor information among the mobile devices by wireless communication; a target focusing section configured to, in response to one of the mobile devices detecting a target, focus a sensor on the target in cooperation with mobile devices adjacent to the mobile device detecting the target; and a target surrounding section configured to surround the target by using a plurality of mobile devices that focus their sensors on the same target.