PURPOSE: To raise the quality of printing by exactly and closely contacting even a larger numbers of pages of printing mediums on the surface of a platen by providing a specific shape of keep plate to a print masking mechanism provided opposedly facing the platen with a spacing.

CONSTITUTION: A pass book(printing medium) 6 is put between a platen 1 and a printing head 2, the printing head 2 is advanced toward the platen 1 side, and while moving the head 2 toward the longitudinal direction, desired letters and marks are printed through an ink ribbon 7. In such a device, a pair of pugh plates 11 to press the book with a given width in the insertion derection in an elastic manner by the back of its one end warped into an arched form are attached to the printing mask 3 of the head 2. Since the book 6 inserted is closely pressed on the platen 1 with a large width by the keep plates 11, clear printing can thus be always attained.