Abstract: A method for measuring the quantity of light emitted from an optical write head which drives an array of light shutter elements extending in a main scanning direction individually in accordance with image data. In order to measure the quantity of light outputted from each of the light shutter elements, a photosensor is moved in the main scanning direction at a constant speed while the light shutter elements are driven in such a way that adjacent elements are not driven at a time. While light shutter elements which are in odd numbers in the array are driven, the photosensor is moved forward to measure the quantities of light outputted therefrom, and while light shutter elements which are in even numbers in the array are driven, the photosensor is moved backward to measure the quantities of light outputted therefrom.