In particular, even if sensors have their respective different timer precisions, the number of occasions when the sensors wait for data can be minimized, thereby reducing the output consumptions on the sensor sides. A coordinator (3) allocates, based on information previously received from sensors (2), the sensors to respective slots, which constitute a super-frame, in a time sequence order and then notifies, to the sensors (2), the respective slots to which the sensors are allocated. When each of the sensors (2) transmits data to the coordinator (3), the sensor makes a synchronization request at the timing of the notified slot without receiving any beacons from the coordinator.