PURPOSE: To test a network system load with high efficiency by controlling the load after subscription of a WS to a LAN without interrupting the test.

CONSTITUTION: A slave WS3nO0 registers its own address into a main WS address register table 120 in a ring subscription mode and knows the contents of the table 120 to register them into a slave WS communication table 3n20. Thus the WS3n0O has the communication with other registered slave WSs.