A first image formation point for a workpiece is provided between a camera and the workpiece. An image of the workpiece is formed at the first image formation point, and picked up by the camera. Meanwhile, an image for a protection glass is not formed at the first image formation point, and is instead diffused and then picked up by the camera. For this reason, an image caused by dirt on the protection glass is diffused. As a result, in the image picked up by the camera, images due to dirt become few and images due to the workpiece become relatively many. Therefore, the accuracy of an evaluation on the quality of the workpiece can be improved.