1. A network system comprising: a plurality of mobile devices each having a moving means and 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 detecting means for reporting whether the target has been detected based on sensor information obtained by one of the mobile devices or for attempting to detect the same target in response to information received that the target has been detected by one of the mobile devices; 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; target surrounding means for surrounding the target by using a plurality of mobile devices that focus their respective sensors on the same target; and wherein the target detecting means describes a maximum value of a number of mobile devices in a packet that reports that the target has been detected on the basis of the obtained sensor information, or ignores a report that the target has been detected from the mobile devices whose number exceeds the maximum value when the report is received.