A heat transfer device for sheet assembly for the heat transfer of an image on a transfer sheet to a receptor sheet for printing by inserting a sheet assembly comprising the transfer sheet on which an image has been formed and a receptor sheet for printing superimposed on an auxiliary plate into an inlet opening portion formed on a casing along an inlet stand, and passing between a pair of heat pressure rolls arranged inside the casing, includes: an exhaust fan for taking the outside air from the inlet opening portion toward the pair of the heat pressure rollers for cooling the pair of the heat pressure rollers; and aperture maintaining means for maintaining air passage for allowing the intake of the outside air between the upper edge portion of the inlet opening portion and the upper surface of the sheet assembly at the time of inserting the sheet assembly into the inlet opening portion. Accordingly, the pair of the heat pressure rollers can be securely cooled by the outside air via the air passage.