PURPOSE: To surely execute monitor and control of a network with simple configuration by providing a code representing a data class such as a command/ response included in monitor control data of the network to each slave station in a unique way respectively.

CONSTITUTION: A master station 1 sends command data having a code representing a data class specific to a slave station 2 to an outgoing line 100, at a point 6f time. The slave station 2 receiving the data detects the code of the data class for itself from the command data, checks the content of the command data and returns response data having a code of a data class specific to the slave station 2 to an incoming line 1002. The master station receiving the returned data detects the code specific to the slave station 2 from the response data to identify it that the response data comes from the slave station 2. Thus, the master station 1 surely identifies a slave station 3 and then monitors and controls the network surely.