PURPOSE: To judge exactly shifted letters, turned letters, omitted letters, etc. printed in a line printer by reading output of doubly printed letters by means of a light receiving element and using obtained wave shapes.

CONSTITUTION: On the rear face of a letter printing paper that is transported by paper-feeding tractors 2 and 3 a hammer unit with a hammer 4 is provided, and on the other hand on the front face a carbon ribbon 6, ribbon separator 7 and a type drum 8 are provided, and furthermore, a reading head that consists of a light receiving element is installed on the upper section of a ribbon separator 7. In the above mentioned printer a same letter is printed doubly and it is detected by the reading head to obtain pulses. The wave shapes of the pulses are compared with the width of a proper pulse to check the quality of printed letters.