Nonuniformity in an organic EL display device is effectively detected. All display pixels of an organic EL panel are turned on and the display is photographed with a digital camera. A computer performs image processing of the photographed image to detect an area in which unevenness exists. Then, a V-I curve of each pixel in the area is measured to calculate necessary correction values. The calculated correction values are stored in a memory for use in correcting a signal input to the organic EL panel.