PURPOSE: To attain fast printing and to obtain a high-resolution multi- gradational image by providing a light emission means consisting of plural light emission parts which output light beams in different spot shapes.

CONSTITUTION: This image forming device is equipped with the light emission parts consisting of a 1st laser diode 101 and a 2nd laser diode 102; and the spot diameter Φ1 of the 1st laser diode 101 is large and the spot diameter Φ2 of the 2nd laser diode 102 is small. Therefore, when a line is plotted by using one dot, a line with the spot diameter Φ1 and a line with the spot diameter Φ2 can be plotted and the thin lines of different size can be plotted to form a sharp image. Consequently, the fast printing is possible and the high-resolution, multi-graduational image can be obtained.