To automatically set a master-slave relationship with a slave unit, to a master unit.

Measurement data is periodically transmitted from the slave unit 1 together with its added identification information (slave unit ID). The master unit 2 is provided with slave unit data storage areas as much as for a maximum number of slave units in which the master-slave relationship between the master unit itself and the slave units can be set. A management device 3 is provided with master-slave relationship storage units for each of master units. The master-slave relationship storage units for each of the master units each have master-slave relationship storage areas as much as for a maximum number of slave units in which master-slave relationship between the master unit itself and the slave units can be set. A master unit 2 once receiving measurement data from the slave unit 1 sends management information including the slave unit ID added to the measurement data to a management device 3. The management device 3 records the slave unit ID of the management information from the master unit 2 in a free area on condition the slave unit ID is recorded in none of master-slave relationship units of master units and the free area is available in the master-slave relationship storage unit of the master unit 2 as the transmission source of the management information, and then determines the master-slave relationship. Further, determination information on the master-slave relationship is sent back to the master unit 2 as the transmission source of the management information.