PURPOSE: To prevent collision between packets due to difference in reservation between communication stations (ground station) and to improve the reliability of communication, by determining allocation of slots on the basis of only information from a control station.

CONSTITUTION: A single communication circuit is divided into small slots 10 with specific time length on time-division basis, a cycle 5 is composed of successive slots 10, and data and reservation slots 8 and 9 are provided to the slots 10 in the cycle 5. A request for reservation allocation of the slot 8 is transmitted from a communication station by the slot 9, the slot 9 is allocated to the communication station on the basis of received reservation information on the whole, and the transmission of information divided by packets is reserved by the slot 8. This communication system is provided with the control station, which accumulates the remaining pieces of reservation information which can not be sent in the cycle 5, sends the accumulation information to other communications stations periodically, and monitors the slot allocation state of the whole. Then, the slot allocation is switched and determined by the information from the control station to prevent collision between packets.