A printing machine includes a transfer device including a conveyor having suction holes formed therethrough for transferring cardboard sheets one by one, a suction device for applying a suction force on one of the two surfaces of each of the cardboard sheets, which one of two surfaces faces the conveyor, and ink jet heads located to face the other of the surfaces of each of the cardboard sheets to be spaced apart therefrom. The ink jet heads are located in such a manner that a desired distance is maintained between tips of the ink jet heads and the other surface to be printed and include ink jet nozzles from each of which ink is jetted out toward said the other surface. The ink jet heads have skirts located upstream and downstream of the transferring direction of the cardboard sheets in such a manner that the skirts extend from the ink jet heads toward the other surface and have a width which covers the ink jet nozzles so as to form a partition in a space between the ink jet nozzles and the other surface.