PURPOSE: To improve the inspection accuracy and to shorten inspection time by deciding the spaces between the edge part of the raster area located at the preliminarily specified position and the reference position.

CONSTITUTION: Each CCD camera 7 performs the image pickup in the edge part in a tester area 8 of a cathod-ray tube device 1 and edge parts 11 of reference boards 6 in camera points 12 where each camera 7 is arranged. A personal computer 15 receives a picture signal and integrates luminance data for each picture element in a picture processing port 16. The brightness of this picture element is integrated in a mask area 18, and the reference boards 6 and a non-display area 10 are turned to black and an area 8 is turned to white so as to be stored in consecutive memory addresses in order. Based on this stored luminance data, a part where the brightness changes rapidly by the influence of edge parts 11 and 9 of the reference boards 6 and the area 8 is detected. Accordingly, the distance between the edge parts 11 and 9 is obtained from the difference of detection values, the inspection accuracy is improved, and the inspection time is shortened.