Ōarai-no-seki was an important facility to supply water in the Kanda aqueduct which used Inokashira-Ike pond as a water source to all around the city of Edo. Water drawn from Inokashira-Ike pond, Zempukuji-Ike pond, etc. was divided into two courses at this dam. Drinking water was first sent to the residence of Mito Tokugawa household and then distributed to all of Edo through the underground. The remaining water was discharged into the Edo River. The facility was built during the governance of the 3rd Shogun Iemitsu.