A multiple access data communication system is disclosed wherein a communication channel is shared by a plurality of stations. The channel is divided into frames each being partitioned into time slots smaller in number than the number of stations of the system. A reservation status signal is constantly broadcast at frame intervals from a central station to all remote stations to indicate reserved status of the time slots. When a transmission request is made in a remote station, it discriminates a message signal shorter than the time slot length as a single packet having a time slot length and detects an idle time slot from the channel using the reservation status signal. If the message signal is longer than the time slot length, the station divides it into a series of packets of the time slot length and transmits a reservation request to the central station to receive from it a time-slot assignment signal and inserts the long-message packets to time slots specified by the time-slot assignment signal.