To perform data transmission by recognizing the line conditions of a communication line of a connected computer network only with an on-line system without using a common protocol and thereby effectively utilizing resources of the communication line and a computer system.

A timer 107 included in the computer system measures the transmission time of data transmission. The measured time is stored in a memory (buffer management table 103) included in the computer system and utilized as a transmission interval for data transmission. Since the measured transmission time changes every moment due to the line use situation of the computer network, the measured transmission time is updated so that the latest situation can be always utilized, and referred to each time data are transmitted to control variably the interval time of data transmission.