勝道上人が急流に阻まれ困っていたところに深沙王が現れ、深沙王が呼び出した蛇が橋になったそうで、勝道上人が渡ったあとはその橋は消えたという話です。
そこに丸太橋を建造して「山菅の蛇橋」と呼んだそうで、神橋の元となった橋だそうです。
今まで何回も架け替えられているそうですね。

Shōdō Shōnin (Saint Shōdō, roughly) was blocked and troubled by the rapid stream when Jinja-ou (Lord of the Deep Sands) showed up. Jinja-ou then summoned his snakes and turned them into a bridge, and the bridge disappeared after Shōdō Shōnin crossed. That's how the legend goes, it seems.

The rustic bridge constructed here is called "Sedge Snake Bridge", and it seems like it was the origin of the sacred bridge.

So far, it seems it has been rebuilt many times.