In a sensor network system, a communications network connects a set of sensors to a server collectively managing the set of sensors. First, the sensor-managing server acquires remaining drive times of batteries in the sensors, and specifies a target remaining drive time. The server then controls the operation of the sensors so that the remaining drive times of the batteries in the sensors are substantially equal to the target remaining drive time. This reduces the maintenance workload for a system manager, especially, in the recharging of the sensor batteries.