In a communication system in which a plurality of communication terminals sequentially transfer data to a server during the respective transmission permissible periods assigned to the respective communication terminals, the communication terminal transfers a preamble for synchronization to the server at a time of starting the transmission permissible period, converts a data frame for every 8 bits into every 10 bit-code, and transmits to the server, the signal string with a code indicating the head added there, during the transmission permissible period, and turns a communication to the server into a zero signal state, during a period other than the transmission permissible period, while the server establishes a synchronization by reading the preamble for synchronization inserted into the signal string received from each of the communication terminals, converts a portion of the zero signal state of the signal string into a predetermined special code string, and inversely transforms the received signal string into the data frame before the coding.