Abstract: Controlling a motor having a high standard frequency in a manner to be given priority when servo-controlling a plurality of motors allows the delay time of motor control to be reduced and the servo control of the motors to be performed with a good response characteristic.

Abstract: A laser printer includes a plurality of lenses and mirrors for scanning a laser beam to form an optical image on a photoconductive drum, a developing unit for storing a developer for developing an electrostatic latent image formed on the drum by the plurality of lenses and mirrors, and an image transfer section for transferring the image developed by the developing unit onto paper. The developer is a two-component developer consisting of a toner and a carrier. The toner densities of toner adhered to a toner hopper for replenishing the toner stored in the developing unit and of that adhered to a toner density detecting probe coaxially attached to the drum are measured by utilizing a light-emitting diode and a photodiode. In a CPU, a toner density measured by the light-emitting diode and the photodiode is compared with a first reference value, and then the measured density value is compared with a second reference value which is smaller than the first reference value.