In the study of the focal trough of panoramic radiograph using Panoura Eight-S (the Yoshida company), a series of 63 X-ray films were taken with the 8-19 metal pins placed in the holes of the plastic model plate and evaluated by 4 observers. The author analized the focal trough defined by the sharpness criteria and calculated dimension of focal trough in the horizontal plane. The results were as follows; 1. Focal trough was not continued in the anterior region within a very high degree of sharpness. 2. With optional sharpness, the width of anterior and posterior focal trough was approximately l3㎜ and 60㎜ respectively. 3. The focal trough was narrow in the anterior region and was flared laterally with symmetry in posterior region. 4. Sharp image began to show at the position of 18㎜ posteriorly from the most anterior position of chin rest.