1. An image forming apparatus, comprising: a platen on which an original is placed so that a first surface of the original is positioned at an exposure area; first detecting means for detecting that a second surface of the original which is opposite to the first surface, has an image; second detecting means for detecting a transmittance of the original; exposure means for exposing the first surface positioned at the exposure area by emitting a light to make a light image; means for adjusting an intensity of the light emitted by the exposure means corresponding to the transmittance when the first detecting means detects that the second surface has the image; and means for forming a copying image on the basis of the light image, wherein the first detecting means includes means for outputting predetermined signals corresponding to a high density area of the second surface of the original, means for counting the number of the predetermined signals, and means for determining that the second surface has the image when the counted number is more than a predetermined number.