PURPOSE: To especially uniformize printing density and to form an image having constant resolving power in an optical writing printer performing optical writing on a photosensitive body using a printing head wherein a plurality of light emitting elements are arranged in an array form.

CONSTITUTION: When image data is written in a frame memory 26, the number of the black dots contained in the image data is counted by a counter/ comparator part 25 to calculate data of a printing ratio. A CPU 20 outputs the corresponding image data to an LED head circuit on the basis of the data of the printing ratio to optically write the same on a photosensitive drum to control an optical writing time. By this constitution, the lights from light emitting elements are uniformized in brightness and image density after the formation of an image becomes constant regardless of the change of the printing ratio.