PURPOSE: To prevent damage resulting from the erroneous operation of a tractor door by connecting a tractor door to a main tractor unit through a linkage mechanism and allowing the door to be operated whilst maintaining it in a parallel position with printing paper.

CONSTITUTION: If a printer cover is opened to set printing paper and rotated against a spring with the operating handle 4 of a linkage mechanism, a tractor door 1 is moved and left open. Perforations on the edge of printing paper are fitted to a projection 8 on a tractor belt 7 stretched on a main tractor unit 2. Then again the operating handle 4 is rotated to close a printer cover, thus completing the setting of printing paper. While setting print paper, the distance between the tractor door 1 and the main tractor unit remains substantially unchanged, even if the door is switched from 'closed' to 'open' state. Accordingly, if an operator by mistake closes the printer cover with the tractor door 1 kept open, the printer cover does not contact the tractor door 1. Thus damage on the tractor door 1 by the printer cover can be avoided.