PURPOSE: To make it possible to detect abnormality of a brake device and abnormal decrease of wire tension by providing such constitution as comparing a tension command value with detected tension to display an alarm message when the detected tension is decreased to a preset proportion or less to the tension command value.

CONSTITUTION: Since a comparator CP is set to an H level when detected tension voltage Tf is decreased to a voltage or less obtained by dividing tension command voltage Tc by resistances R10, R11, with no normal operation of an electromagnetic brake device B, an output signal is output to a numerical control unit by actuating the light emitting diode 3-1 of a photocoupler 3 and turning on a light receiving element 3-2. When the numerical control unit receives this signal, it displays an alarm message of brake device abnormality on a CRT display device. In this way, work of inspection or replacement of the electromagnetic brake device B is performed by an operator based on this display.