This study aims to clarify the spatial structure of the “Mt. Iwaki belief” in the Tsugaru district of Aomori Prefecture. During the Edo period Mt. Iwaki belief was encouraged by the Tsugaru-han, the local authority in the district, who supported Hyakutakuji temple on Mt. Iwaki in order to further their political control. The han authorities also accepted the local Tango-Biyori legend which suggested that whenever people from the Tango region of Kyoto prefecture entered Tsugaru the wrath of Mt. Iwaki was incurred and stormy weather would result. In times of bad weather, therefore, the people of Tsugaru would hunt down anyone hailing from Tango. Given the tacit encouragement of the authorities, it is fair to suggest that the sphere of Mt. Iwaki belief covered the same area as the territory of Tsugaru-han.This said, it should also be pointed out that neither the han authorities, nor Hyakutakuji temple, had direct control over Mt. Iwaki belief. Thus, Oyama-Sankei, one of the central features of Mt. Iwaki belief, was never controlled by any particular religious organization. Oyama-Sankei was a coming-of-age initiation ceremony held on the lunar equivalent of August 1st. The age at which initiants took part, however, varied among the different villages of Tsugaru. In the 15 kilometer zone surrounding Mt. Iwaki, initiation took place during young childhood. Further away, in the 15-30 kilometer zone, initiation was at a slightly older age, and in the furthest zone, 30-70 kilometers away, initiation was delayed until adolescence. In this outer zone it was also the case that other mountains were used as a substitute for Mt. Iwaki.There was no clearly defined route for pilgrims observing Oyama-Sankei, and a number of ways up Mt. Iwaki were utilized. Four routes stand out in particular, passing through Hyakusawa-guchi (at the south-east foot of the mountain), Nagadai-guchi (north-west), Dake-guchi (south-west), and Oishi-guchi (north-east). The former two were used by locals as well as those from further afield, and the latter two mostly by locals. Thus, the spatial structure of Mt. Iwaki belief can be considered from the point of view of age of worshippers at the time of initiation, the route used for pilgrimage, and the location of substitute mountains.