[en] There are many factors affecting the quality of radiographic images in radiographic tests. In this experiment, as a part of the tests, the detectability of planar defects was examined using slit test pieces on the effect due to the geometrical conditions in radiography. The slit test pieces have 3 parallel slits at 6 mm intervals. Employing 3 test pieces as a set, radiograms were taken placing the test pieces and a film on an iron plate placed 110 mm high over the floor, laying a lead plate of 2 mm thickness on the floor of the testing room. The test pieces were made of steel plates of 100 x 300 x t mm, t being 15, 20, 25 and 40 mm. Major photographing conditions were as follows. Distance between the radiation source and the film was 200, 300, 400, 500, and 600 mm. Developing conditions were tank development at 20 deg.C for 5 minutes and density 2.2 +- 0.3. These experimental results are described about the detectability of the slit width in relation to the distance between the source and films, and that of planar defects relative to the position of test pieces and the angle of incident radiation. In short, image quality is better as the distance between the source and films is larger, as the position of the test pieces is nearer to the film, and as the angle of incident radiation is smaller, within the range of thickness of the test pieces employed. (Wakatsuki, Y.)