A master device, the master device comprising: a storage means for storing setting information, in which information on the devices constituting a network system to which the master device belongs as well as information on the topology of these devices is set, the network system including, in addition to the master device, a plurality of slave devices, wherein the master device is configured to communicate with the slave devices using a position address and a node address and a logical address, which are three kinds of addresses of a slave circuit included in a hub device of the network system, wherein the position address is an address that is decided by a physical connection sequence of the slave circuit when viewed from the master device, said setting information indicating a connection sequence of the slave circuit, the node address is an address that is allocated to each slave circuit irrespective of the physical connection sequence of the slave circuit from the master device, and the logical address specifies the position of each piece of data to be transmitted by or received by the master device and each slave circuit, the network system further including the hub device comprising a port to be connected to an upstream device and a plurality of ports (OUT1 - OUT6) to be connected to downstream devices; a real configuration information generating means configured to generate real configuration information based on information collected from each device in the network system, the real configuration information comprising information on the devices constituting the network system and topological information on the devices; and a determination means configured to determine whether a connection of each device in the network system is correct by comparing the real configuration information to the setting information, wherein the determination means determines that the connection of a slave device connected to a downstream port (OUT1 - OUT6) of the hub device is correct even though the port (OUT1 - OUT6) to which the slave device is connected is incorrect, if the real configuration information and the setting information both denote the same hub device to which the slave device is connected. a storage means for storing setting information, in which information on the devices constituting a network system to which the master device belongs as well as information on the topology of these devices is set, the network system including, in addition to the master device, a plurality of slave devices, wherein the master device is configured to communicate with the slave devices using a position address and a node address and a logical address, which are three kinds of addresses of a slave circuit included in a hub device of the network system, wherein the position address is an address that is decided by a physical connection sequence of the slave circuit when viewed from the master device, said setting information indicating a connection sequence of the slave circuit, the node address is an address that is allocated to each slave circuit irrespective of the physical connection sequence of the slave circuit from the master device, and the logical address specifies the position of each piece of data to be transmitted by or received by the master device and each slave circuit, the network system further including the hub device comprising a port to be connected to an upstream device and a plurality of ports (OUT1 - OUT6) to be connected to downstream devices; a real configuration information generating means configured to generate real configuration information based on information collected from each device in the network system, the real configuration information comprising information on the devices constituting the network system and topological information on the devices; and a determination means configured to determine whether a connection of each device in the network system is correct by comparing the real configuration information to the setting information, wherein the determination means determines that the connection of a slave device connected to a downstream port (OUT1 - OUT6) of the hub device is correct even though the port (OUT1 - OUT6) to which the slave device is connected is incorrect, if the real configuration information and the setting information both denote the same hub device to which the slave device is connected.