For large scale disaster relief, we propose a location and short message system using the Quasi-Zenith Satellite System (QZSS). The target capacity of the proposed system is the transmission of 3 million message data (MD) or more within one hour. To control random access of mobile terminal (MT), Frame Slotted ALOHA is used. Frame Slotted ALOHA cannot provide optimum control, since the hub station (HUB) does not know the number of MTs. For realizing large capacity, we propose Frame Slotted ALOHA with Flag Method in which the HUB can estimate the number of MT quickly. The proposed Flag Method is divided into two modes. The send flag mode can estimate the number of MT within a short time. The send message data mode can control random packet access using estimated number of MD in send flag mode. Computer simulation shows that it is possible to accommodate 3 million MD by using the proposed Flag Method within 48m 19s.