A yellow luminous-element array, which is an optical fixing unit, includes a large number of yellow LEDs emitting ultraviolet rays. The LEDs in each of lines extending in a feeding direction are connected in series. Integral illuminance of each line is adjusted by changing electrical energy to be supplied to the respective lines. The integral illuminance of each line extending in the feeding direction is measured to obtain illuminance distribution relative to a scanning direction when the yellow luminous-element array is examined at the time of manufacture thereof. The integral illuminance of each line is corrected so as to even the illuminance distribution.