An exposure system is provided with a light emitting element array formed by a plurality of light emitting elements formed at the intersections of anodes and cathodes arranged in matrix so that a photosensitive material is exposed to an image formed on the light emitting element array. A method of driving the light emitting element array includes the steps of driving the light emitting elements in constant-current drive before and/or during an exposure period, measuring and storing in a memory means the anode voltage of each light emitting element at that time, and driving the light emitting elements in constant-voltage drive at a voltage equal to the measured anode voltage at least during beginning of the subsequent exposure periods.