PURPOSE: To properly correct the quantity of light emission of each LED in the LED array head for half-tone image data by extracting correction image data from input image data according to the reference quantity of light emission and outputting the data to a specified LED.

CONSTITUTION: Each of the LEDs in the LED array head 6 is previously supplied with a current having a level corresponding to the maximum value of gradations and the reference quantity of light emission of each of the LED is measured. Then the mean value of the light emission quantities is found and the rates of the respective LEDs to the mean value are stored in a dot data storage part 3. When the image data are outputted to the respective LEDs, a data correction part 4 refers to a data table and outputs the correction data to a head controller 5. Then the respective LEDs emits the quantities of light corresponding to the respective correction data to expose a photosensitive body. The corrections are so made that the image data are large for the LEDs which are low in rate of the quantity of light emission and small for the LEDs which are large in rate, so variance in the quantity of light emission is suppressed to reduce light and shade unevenness.