SERIAL PRINTER

PURPOSE:To prevent the positional shift of a printing medium due to the movement of a printing head, by a method wherein a guide roller is provided to a printing head and a pair of thickness detection plates are arranged between a guide roller and a printing medium, and a notch part is provided to the lower part of the leading end of each of the detection plates. CONSTITUTION:A pair of thickness detection plates 9A, 9B arranged between the guide roller 3 provided to a printing head 1 and a printing medium 2 are arranged so that the leading ends thereof are superposed at the binding part 2a of the printing medium 2 and the rear end parts thereof are energized in a direction separated from the printing medium 2 by the spring 11 pivotally supported by a shaft 10 in a freely revolvable manner. Notch parts 12 are respectively provided to the lower parts of the leading ends of the thickness detection plates 9A, 9B and the thickness difference generated at a center holding part 2a is absorbed by the notch parts 12. The leading ends of the thickness detection plates 9A, 9B are inclined along the moving direction of the printing head 1 and the directions of inclination thereof are made reverse. Since the printing can be performed while the thickness of the printing medium 2 in a printing direction is detected and the printing medium 2 is pressed between a platen 5 and the thickness detection plates 9A, 9B, the positional shift of the printing medium 2 due to the movement of the printing head 1 can be prevented.