1. A network system comprising: a plurality of mobile devices each having a sensor function and a wireless communication function, the mobile devices communicating sensor information about a target with one another and tracking the target; target searching means for obtaining sensor information while the plurality of the mobile devices move, and for reporting the sensor information among the respective mobile devices by wireless communication; target focusing means for, in response to one of the mobile devices detecting a target, focusing a sensor on the target in cooperation with respective mobile devices adjacent to the mobile device detecting said target; and target surrounding means for surrounding the target by using a plurality of mobile devices that focus their respective sensors on the same target.