PURPOSE: To prevent an accident by stopping a line printer on detecting a word omission or the breaking of a print band before it occurs due to abnormal abrasion of an anvil.

CONSTITUTION: When the back surface of print band 3 contacts the tip of projection 6 in the strike of hammer 1 with anvil 2 in an abrasive state, abnormal frictional force acts between the both and print band 3 changes in speed. This projection 6 provides the same effect without reference to its length in the traveling direction of print band 3. If the speed change exceeds an allowable value, a band speed check signal is generated to stop the line printer.