In an image forming apparatus, an object surface (14) is disposed facing one end face of a rod lens array (10), while an image surface (16) is disposed facing the other end face thereof. A lens working distance of the rod lens array on the object side is substantially equal to that on the image side. An actual object-image distance Tco is set between the conjugate length TC1 at which the average value MTFave of the MTF of the rod lens array in the lens array direction is maximized and the conjugate length TC2 at which the &Dgr;MTF(=(MTFmax−MTFmin)/MTFave) is minimized, and a shift quantity &Dgr;TC(=|TCo−TC1|) is set within 0 mm