After following the road north for several minutes, you will find yourself in the center of town. Once you see a stone torii gate to your right, stop—you have reached the Onsen Jinja, or Hot Spring Shrine.
Founded in 701 as Shimengū, the shrine was originally a part of the same temple complex as Manmyōji. This is the head shrine of Oshimen, a four-faced god worshipped across the Shimabara Peninsula as a protector of the region and its hot springs.
Shimengū was affected by another period of religious tension in Unzen. During the second half of the nineteenth century, the new Meiji government sought to undo centuries of tradition by which Buddhist deities were interpreted as Shinto gods and worshipped together—often in the same place. Shimengū was forcibly separated from the rest of Manmyōji and given a new name to separate it from what was then seen as the “foreign” Buddhist influence.
After World War II, its original name was restored, and locals tend to also call it by its nickname, Oshimen-san. The physical separation, however, was never corrected.