To make an unpainted surface surely printable, regardless of the front or rear side, that is, direction of the printing surface of a placed printing sheet, when the printing sheet printed on one side is used, in a printer for printing the printing sheet.

It is judged whether the printing surface of the printing paper supplied into a developing unit as a printing means is already printed or not (S4). When the printing paper is the printed paper, the printing surface of the paper is turned upside down (S5) and printed (S6). When the printing paper is not the printed paper, printing is executed as it is (S6). After that, the paper is ejected (S7). Thus, the unprinted surface of the paper can be surely printed and a contribution to the effective/practical use of resources can be made. Then, printing efficiency is increased and further, it is unnecessary that a user places the printing sheet, with the consciousness of the direction of the printing surface, when he places the printing paper.