To prevent not only surface stains but also image quality deterioration owing to a back printing image by adopting an intermediate position between an image peak position and a sight through peak position as a threshold value to process density adjustment in an image density adjusting means.

An original density is detected by an optical sensor 36. A histogram-generating means 43 generates a histogram related to an original image density distribution. A peak position detecting means 44 compares the adjacent frequency values in the histogram stored in a counting circuit 42, so as to detect the peak position of the histogram. The image part, the sight through part and the surface part of the original image density are detected from the peak-detecting position. A density-adjusting means 45 executes a binary processing with an intermediate density between the image part and the back printing part as the threshold value and gives image data after density adjustment to an optical write unit 6. The images with the density being lower than that of the sight through part are not reproduced on recording paper.