PURPOSE: To shorten the transmission time by varying the frame length of transmission data in accordance with the quality of a transmission line.

CONSTITUTION: Effective transmission bits B are considered with a frame length (f) and a check code length (n) as parameters to obtain equations. Theses equations are graphed as shown in the figure. On this graph, the quantity of transmission data is larger on a curve B1 when the error frequency is lower with an error rate e0 as the boundary, and the quantity of transmission data is larger on a curve B2 when the error frequency is higher. A line is connected and a maximum transmission speed where both transmission/reception equipments can communicate is set. A test signal is transmitted thereafter and the line quality is detected in accordance with the response signal from the reception side. The block length is determined based on the detected line quality, and data is transmitted at the determined transmission speed with the determined block length.