The roentgenologic diagnosis of lateral subluxation of the atlanto-axial joint is very difficult because the only presentation is increase of the atlanto-dental interval. This study was carried out with 70 volunteers for normal value of atlanto-dental interval. We measured these intervals from lateral roentagenograms of cervical spine in neutral, flexion, and extension position position of the neck. The results were as follows : 1. The mean value of atlanto-dental interval in all subjects was 1.54$pm$0.52 mm in neutral, 1.59$pm$0.62mm in flexion, and 1.46$pm$0.48mm in extension position. 2 . After thirty years of age the atlanto-dental intervla was getting narrower according to aging. 3. In neutral and flexion positions there is no difference in atlanto-dental intervals. but in extension position it was significantly narrowed.