An image forming apparatus includes a printing unit for printing an image on a paper, a paper feed mechanism for feeding a paper to said printing unit, a paper supply cassette for forwarding papers, one by one, to said paper feed mechanism, an optical sensor for detecting whether or not a surface of each paper, fed to said printing unit by said paper feed mechanism, is blank, and an ejecting unit for ejecting a paper from said paper feed mechanism without printing an image thereon when said optical sensor detects that the surface of the paper is not blank, so that only papers having blank surfaces are fed to said printing unit.