Abstract: In a bus type LAN, the data transmitted from a terminal (14) is detected by an adapter (1) connected to one end of a bus (3), and in response thereto the adapter (1) transmits a pulse signal onto the bus (3). An adapter (2) connected to the other end of the bus (3) receives the data transmitted from the terminal (14) and detects its source terminal address. A difference in time between the arrival of the data from the terminal (14) and the arrival of the pulse signal from the adapter (1) is measured by a counter circuit (21), and the count value obtained is stored in a memory (23), forming a pair with the corresponding source terminal address. Thus, the count value can be obtained for each terminal during the normal operation of the LAN. The data of the memory (23) are extracted by a personal computer (5), where the physical position of each terminal is calculated in accordance with the bus propagation velocity, and is displayed or printed out as a physical constitution diagram.