PURPOSE: To shorten the time required for adjustment and to eliminate the waste of paper by changing an optical condition at every specified interval of an original for adjustment, reading image data and forming an image.

CONSTITUTION: When an operator inputs a code corresponding to a dimming control mode by using ten keys 33 and turns on a copying key 31, 'ORG' designating that a sheet for adjustment is placed is displayed on a liquid crystal display part 34 by a control part 71. Next, the operator places the sheet for adjustment on an original platen and turns on the copying key 31, whereby image forming operation is performed while controlling the lighting of the exposing lamp 4 at exposing voltage which is gradually decreased by one volt in every area by the control part 71. Thereafter, 'AJV' designating that a corresponding adjusted value is inputted is displayed on the display part 34 by the control part 71. Then, the operator inputs the optimum adjusted value judged based on a printed image by using the ten keys 33 and turns on the copying key 31. The control part 71 receives the adjusted value and stores it in a RAM 71c.