Purpose: A panoramic TMJ projection for lateral views has been reported as very useful for the observation of the condylar position in the fossa. We reported the visibility of bony outlines on lateral panoramic TMJ images with an AZ3000^TM rotational panoramic X-ray unit (ASAHI Roentgen Industry Company, Limited, Kyoto, JAPAN) in a previous study (part 4). In this study, we investigated the optimal image layer thickness to improve the visibility of the TMJ images. Material and Method: We altered the AZ3000^TM so that we could adjust the width of slit (named AZ3000TU) to change the image layer thickness. The image layer thicknesses at various combinations of the first and second slits were defined with a wire mesh phantom. A tissue equivalent phantom was positioned and the panoramic lateral TMJ projection was taken with different image layer thickness. The phantom shifted 2.5-5mm inside and outside from standard position in the horizontal plane to simulate miss-positioning of the patient using various combinations of the slits. Four sites, postglenoid spine, glenoid fossa, articular eminence and condyle, in the radiographs were evaluated to find out the best slit combination and whether the sites could be observed or not by two oral radiologists. Results: The image layer thickness was 6mm at 1.2/8mm slit combination, 8mm at 1.0/7mm, 11mm at 0.5/5mm and 25mm at standard lateral mode. When the image layer thickness was at 6, 8 and 11mm, the four sites of TMJ were all observed. At 25mm image layer thickness, the mid cranial base superimposed TMJ strongly. In the case of 6mm-image layer thick ness, by positioning the phantom 5mm lateral from the standard position in the horizontal plane the postglenoid spine was difficult to observe in the radiograph. Conclusion: In the radiographs at the standard position where the image layer was less than 11mm, all four sites of TMJ were observed. However, at 6mm image layer thickness (the thinnest), when the object was positioned 5 mm lateral from the standard position, the postglenoid spine was difficult to detect.