PURPOSE: To inspect a defect in the original plate set to an exposing device, at any time by reading the image of the original plate at the time of replacing a work to the exposing device, and discriminating the defect in the original plate based on the information of the output of the read image.

CONSTITUTION: A light source 1 emits light, and irradiation with the light us executed from the direction perpendicular to the mask holder 4 via a fly eye lens 2 and a reflection mirror 3 to transfer the image of the original plate 6 on the work 15. After an exposure, a work holder 5 is lowered, and then the work finished to expose 15 is sucked by a work carrying device 16, and transferred to the next process. After that, the next work 15 is carried up to the work holder 5 in prescribed order, and placed on the top surface part. At this time, an image reader 12 inspects the defect in the original plate 6 through the use of a work replacing time. When the defect in the original plate is displayed on a display part, the work carrying device 16 is transiently stopped, and the original plate 6 is cleaned, or moved from the mask holder 4 to be replaced with another original plate finished to inspect.