Koshinsha is a site dedicated to the Koshin faith. Origiunally Chinese Taoist in essence it was introduced into japan from Korea in the 8th Century.
Adherents stay awake all night every 60 days on Ko Shin days to stop 3 "worms" from leaving their body and reporting to a god about their good and bad behaviour.
Koshin faith became influenced by Buddhism, and in the middle of the Edo period by a branch of Confucian Shinto that equated the faith with Sarutahiko. In Meiji it was discredited as "superstition" though it has survived a little. There is also a small Inari shrine in the grounds.
At some point in its history the symbolism of the three monkeys became a part of it, and that is its most common identity today.