To widen the range of objects to be checked if the tapping state is acceptable or not and facilitate restoration work by a worker and smooth tapping work by clarifying the whole of the tapping state while before, during and after the tapping work.

The operator checks if the tapping state is normal or abnormal after finishing the tapping work. When it is abnormal, the operator recognizes the abnormal state, the operator stops the machine by an emergency stop to stop the tapping work in all cases irrespective of the mode of the abnormal state thus recognized, or, according to the mode of the abnormal state recognized, the operator stops the machine by an emergency stop to stop the tapping work and resume the tapping work.