According to local tradition, a maidservant employed by a household in Onuma worked daily in the surrounding fields. During her breaks, she would comb her hair while gazing into the still waters of the marsh, using its mirrored surface as her reflection. One day, she failed to return home. When villagers searched for her, they found her sandals neatly placed at the water’s edge. It was believed that she had been taken beneath the waters by the master spirit of the marsh.

That night, the household head, Gizaemon, dreamt that a deity appeared beside his pillow, telling him: “Come to the marsh at midnight, and you shall receive a treasure.” Overcome by fear, he did not go. Soon after, a great surge of water is said to have burst into his home. Thereafter, the spirit of the upper marsh was believed to have moved to what is now venerated as the pond of Oyodo Daimyojin.

The sacred embodiment (goshintai) of the deity is variously described as a blue serpent (aodaisho) or, according to other accounts, as a great carp seen in the waters. In later generations, local people prayed at the shrine to protect their silkworms from being devoured by mice, reciting: “Show yourself not, and consume the mice instead.”

It is also said that the marsh deity later relocated from Onuma to a southern pond, and that the drag marks left at that time gave rise to the place name “Numazuri.”