PURPOSE: To provide an abnormality monitoring method for detecting an acci dent such as a collision generated during the ATC action of a machine tool in a machining center or the like before getting to such a large accident as to damage the machine so as to alarm and bring it to a stop.

CONSTITUTION: The current value of an ATC motor 3 during the continuous ATC action of a tool changing arm 1 gripping a tool, pulling it out, turning it by 18° and inserting it is measured every assigned tool to prepare plural current value data for storage. The current data of the assigned tool is called by a tool change command and multiplied by 1. 5, for instance, to obtain allowable value data for storage. The current measured value of the ATC motor during operation is compared with the corresponding allowable value of the allowable value data, and alarm-stop is performed at the time of exceeding the allowable value.