A program recording medium recording a communication data transmission program for a communication data transmission equipment which sends and receives data to and from opposite transmission equipment connected via a long-distance optical fiber circuit, said program comprising procedures of:
- sending data before arrival of an acknowledgement reply procedure, after sending a connection establishment request to said opposite transmission equipment;
- acquiring data loss occurrence probability by estimating from data loss occurrence state in sending and receiving of data or by obtaining from said opposite transmission equipment;
- and
- performing control not to restrict a data size per each sending of data until said data loss occurrence probability exceeds a predetermined value, and to reduce the data size per each sending of data according to an increase of the data loss occurrence probability in case said data loss occurrence probability exceeds the predetermined value.