A data communication method able to transfer data well in real time group communication between one and many communication terminals, which has a communication server system repeatedly judge whether or not each receiving side communication terminal can communicate, postpone the transfer of data to the communication terminals when it judges there is a communication terminal which cannot communicate, and newly transfer the data postponed in transfer to all of the receiving side communication terminals when subsequently judging that the communication terminal judged to be unable to communicate is able to communicate and a communication server system and a communication terminal used in the same.