An EL layer is located between a linear electrode and an opposite electrode, which stands facing the linear electrode, either one of the linear electrode and the opposite electrode being constituted of a light transmissive electrode. Electric power is supplied across the EL layer, which is located between the linear electrode and the opposite electrode, via the linear electrode and the opposite electrode. An optical intensity of light radiated out from the EL layer, across which the electric power has been supplied, is detected. The electric power, which is supplied across the EL layer, is controlled such that the thus detected optical intensity and a predetermined optical intensity, which has been set previously, become approximately identical with each other.