Based on the attenuation characteristics of rays (X-rays, γ-rays and neutron rays) when they propagate in the medium, when the rays with uniform intensity are injected into the inspected part from one side, the intensity of the rays will be uneven after passing through the inspected part due to the different attenuation characteristics of the defects and the substrate materials of the inspected part. By detecting the intensity of rays passing through the inspected part on the opposite side through film photography and direct observation on the fluorescent screen, it can be judged whether there are defects (uneven spots) on the surface or inside of the inspected part.