A control device, suitable for an industry network and factory automation system comprising a master device and a plurality of slave devices (20a-20d), the control device being the master device, wherein the slave device (20a~20d) comprises: a first communication unit connected with a first another device on a side of the master device by a first communication cable (30a~30d) and a second communication unit connected with a second another device on a side opposite to the master device by a second communication cable (30a-30d), the first communication unit further comprises a first detection unit and a first recording unit, provided in the first communication unit, wherein the first detection unit performs a breakdown diagnosis comprising detection of a breakdown location of the first communication cable (30a-30d), and the first recording unit keeps a result of the breakdown diagnosis performed by the first detection unit; and the second communication unit further comprises a second detection unit and a second recording unit, provided in the second communication unit, wherein the second detection unit performs a breakdown diagnosis comprising detection of a breakdown location of the second communication cable (30a-30d), and the second recording unit keeps a result of the breakdown diagnosis performed by the second detection unit; and characterized in that the control device comprises: a communication instruction unit, instructing the slave device (20a~20d) to execute/stop communication with the first another device or the second another device a diagnosis instruction unit, instructing the slave device (20a~20d) to implement the breakdown diagnosis of the first communication cable (30a~30d) or the second communication cable (30a-30d); a diagnostic result reading unit, reading out a result of the breakdown diagnosis from the first recording unit or the second recording unit of the slave device; and a calculation unit, calculating a length of the first communication cable (30a~30d) or the second communication cable (30a~30d) from the slave device (20a~20d) to the breakdown location from the result of the breakdown diagnosis that is read out. a first communication unit connected with a first another device on a side of the master device by a first communication cable (30a~30d) and a second communication unit connected with a second another device on a side opposite to the master device by a second communication cable (30a-30d), the first communication unit further comprises a first detection unit and a first recording unit, provided in the first communication unit, wherein the first detection unit performs a breakdown diagnosis comprising detection of a breakdown location of the first communication cable (30a-30d), and the first recording unit keeps a result of the breakdown diagnosis performed by the first detection unit; and the second communication unit further comprises a second detection unit and a second recording unit, provided in the second communication unit, wherein the second detection unit performs a breakdown diagnosis comprising detection of a breakdown location of the second communication cable (30a-30d), and the second recording unit keeps a result of the breakdown diagnosis performed by the second detection unit; and characterized in that the control device comprises: a communication instruction unit, instructing the slave device (20a~20d) to execute/stop communication with the first another device or the second another device a diagnosis instruction unit, instructing the slave device (20a~20d) to implement the breakdown diagnosis of the first communication cable (30a~30d) or the second communication cable (30a-30d); a diagnostic result reading unit, reading out a result of the breakdown diagnosis from the first recording unit or the second recording unit of the slave device; and a calculation unit, calculating a length of the first communication cable (30a~30d) or the second communication cable (30a~30d) from the slave device (20a~20d) to the breakdown location from the result of the breakdown diagnosis that is read out. a communication instruction unit, instructing the slave device (20a~20d) to execute/stop communication with the first another device or the second another device a diagnosis instruction unit, instructing the slave device (20a~20d) to implement the breakdown diagnosis of the first communication cable (30a~30d) or the second communication cable (30a-30d); a diagnostic result reading unit, reading out a result of the breakdown diagnosis from the first recording unit or the second recording unit of the slave device; and a calculation unit, calculating a length of the first communication cable (30a~30d) or the second communication cable (30a~30d) from the slave device (20a~20d) to the breakdown location from the result of the breakdown diagnosis that is read out.