To prevent the degradation of a communication performance by performing the return of acknowledgement(ACK) at suitable timing.

In the case of starting communication between respective communication end points 110A and 110B of computer systems 100A and 100B, a buffer size reporting part 120A (120B) reports the maximum capacity of a transmission/ reception buffer 112A (112B) to the computer system 100B (100A). A buffer size receiving part 121B (121A) stores the reported maximum capacity in a buffer size storage part 11B (111A). An ACK return timing calculating part 140B (140A) discriminates whether the amount of data stored in the transmission/reception buffer 112B (112A) satisfies a prescribed ratio to the maximum capacity of the transmission/reception buffer 112B (112A) or maximum capacity of the transmission/reception buffer 112A (112B) or not or whether it is after the lapse of prescribed time from the last ACK return or not.