PURPOSE: To provide an image density control method for correcting the reduction in light quantity caused by the soiling of an optical system with a simple structure.

CONSTITUTION: In view of the fact that when an original density sensor 18 is arranged in the optical system, the soiling due to the aging of the optical system is equal to that of the original density sensor 18, the number of copies is counted at the time of copying, the surface density of each original with respect to the prescribed number of copies is detected by the original density sensor 18 at the constant intervals of the count value and these detection voltages Vd are added in each of the prescribed number of copies, to calculate an average value Xo. Then, fluctuations in the light quantity of the optical system are detected from a change in the average value Xo in each of the prescribed number of copies and the lamp voltage of an exposing lamp 7 is controlled to correct according to the fluctuations in the light quantity, so that an optimum density image corrected in the decrease of the light quantity caused by the soiling of the optical system with the lapse of time can be obtained.