PURPOSE: To easily estimate the state of lines in use, by obtaining the ratio of time between the line in use consisting of sum of an information frame and response frame and the line in non-use being flowed with a gore head frame, at a line using rate detection section.

CONSTITUTION: In a reception control section, a data reception signal, a flag reception signal, a gore head frame GA reception signal are distinguished in byte unit and outputted. The logical sum between a data reception signal 26 and a flag reception signal 27 out of the signals is summed in a counter 21. The logical sum with a GA signal 28 is summed to another counter 20 and an overflow signal is outputted at a favorable multiple of the counter 20 and the content summed in the counter 21 is set to a register 22. Just after the operation, the counters 20 and 21 are cleared for the preparation to the next reception signal. Thus, since a new line using rate enters the register 22 repetitively, the content is displayed on a display 23.