PURPOSE: To detect the area of disconnection within a control board by detecting disconnection of a signal line through the presence or absence of a monitor signal sent from a monitor terminal as well as the disconnection of the corresponding branch signal line from the monitor data of a monitor signal.

CONSTITUTION: If the disconnection arises at a point X on a signal line 4, it is impossible for a main control board 2 to receive the monitor signals sent from the monitor terminals 12∼ set after the point X. Therefore the board 2 decides that the signal line 4 following a monitor terminal device 11 has the disconnection. In case the disconnection arises at a point Y of a branch signal line 4' within a control board A1, the control signal sent from terminals 11, 12∼ can be received by the board 2. Thus the board 2 understands no disconnection arises on the line 4. At the same time, it is known from the monitor data contained in the monitor signal of the device 11 that the monitor input is set at an L level. Therefore the board 2 can detect that the disconnection arises on the line 4' within the board A1.