PURPOSE: To reduce the quantity of information on the multi-address tracing operation by excluding the duplex frames like the UP frames, etc., caused by the retry carried out at the multi-address communication out of the trace collecting objects.

CONSTITUTION: A trace function part 6 reads the trace information out of the position of a trace buffer 7 pointed by a pointer 8 and checks whether the frame transmitted and received right before read-out of the trace information is coincident on not with the frame transmitted and received this time and also with a prescribed frame of a different type, e.g., a transmission right transfer frame (UP frame). When so, it is decided that the frame transmitted and received this time is equal to that caused by the retry and no trace information is produced. When not, the trace information is produced and +1 is given to the contents of the pointer 8 to write the produced trace information. Thus it is possible to prevent the increase of the trace information quantity caused by the repetition of such a frame as that caused by the retry of the UP frame having no relation with the actual processing.