A control unit for a machine tool at least one vibration detection device attached to a main shaft portion of a machine tool or a movable body; a storage device for storing a vibration signal detected by the vibration detection device being related to operation information of the machine tool; a collision judgment device for judging that a collision has occurred; and an output device for outputting the vibration signal at the time of judging the occurring of the collision. By the above constitution, the control unit is capable of grasping a specific circumstance of collision and quickly determining a cause of collision.