PURPOSE: To prevent a printing head from printing on the outside of the width of a paper and prevent the paper from jamming, by providing a sensor for detecting the movement of the printing head slightly to the inside of a guide for the paper on a sheet feeder, and setting the detected position to be a margin position.

CONSTITUTION: A magnetic sensor 4 provided on a printing head 3 detects the magnetism of the magnet 2 when it is moved to the front side of the magnet 2, thereby detecting that the head 3 is moved to a limit position of a printing range of a paper 6, namely, a margin position. The position of detection of the left magnet 2 by the magnetic sensor 4 is set to be a left margin, while the position of detection of the right magnet 2 is set to be a right margin, detection signals are sent from the sensor 4 to a controlling part of a serial printer, and the movement of the printing head 3 is controlled, whereby it is possible to prevent printing from being performed on the outside of the left margin or the right margin.