PURPOSE: To attain easy and accurate analysis of a cause to a communication fault in an SDLC loop communication.

CONSTITUTION: A primary station 1 is provided with a memory (part of area of RAM) 71 storing data for fault analysis as well as a common memory (RAM) 7 storing communication information. When the primary station receives a frame being an acknowledge from any of secondary stations 3, 4...5 and an information field (I part) is in existence in the frame, data in the information field (I part) are written in a common memory and data of an address field (A part) and data of a control field (C part) are written in a fault analysis memory. The write processing as above is implemented till a fault takes place and the SDLC communication is stopped.