PURPOSE: To prevent the pressing force due to the deflections of heat rollers from lowering by a simple structure.

CONSTITUTION: On a guide plate 32, on the sides of which image sheet 26 and image receiving sheet 28 are placed respectively, convex surfaces, which makes the thickness of the guide plate gradually thicker in the direction of the axis lines of heat transferring rollers from the thicknesses (d) at both end parts of the rollers toward their axial middle parts, are made. The change of the thickness in the axial direction of the heat transferring rollers of the guide plate 32 corresponds to the amounts of deflection of the heat transferring rollers under the condition that the predetermined pressure are applied to the journal end parts of the heat transferring rollers. Thus, to the heat transferring rollers, between which and the guide plate 32 the image sheet and the image receiving sheet are pinched respectively, nearly uniform same stresses as the stresses F applied at the journal ends of the heat transferring rollers are acted axially, resulting in developing no decrease of pressing force between the image sheet and the image receiving sheet.