Laser beam printer capable of forming line images having stable image width

A laser beam printer comprising:

• forming means for electrostatically forming a latent image on a surface of a photosensitive member by a laser beam;
• developing means for developing said electrostatic latent image formed on the surface of the photosensitive member;
• applying means for applying a bias voltage to said developing means;
• control means for controlling the forming means so as to form at least one latent reference line image on the surface of the photosensitive member and further controlling the developing means so as to develop said latent reference line image to make a developed reference line image;
• detecting means for detecting a width of said developed reference line image, said detecting means including a photoemitter element, a photoreceptor element and a slit;
• counting means for counting the time during which the developed reference line image passes through the slit to determine the width of the developed reference line image;
• storing means for storing a reference image width in a memory;
• comparing means for comparing the detected image width with the reference image width;
• and
• varying means for varying a value of the bias voltage applied to the developing means in accordance with the difference between the reference image width and the detected image width.