Kosui-sai (Lake Water Festival – Eve Ceremony)

A Sacred Celebration of Tradition, Nature, and Light on the Waters of Lake Ashi

A beloved summer tradition in Hakone, the Kosui-sai (Lake Water Festival – Eve Ceremony) is a solemn Shinto ritual held on the evening before the grand annual festival of Hakone Shrine. Following the sacred “Ritual on the Shrine Grounds,” a ceremonial boat (御供船 osonaebune) sets sail across Lake Ashi. There, offerings are made to the guardian deity of the lake, Kuzuryu Okami, during the “Ritual on the Lake.” As the ritual concludes, thousands of fireworks light up the night sky over Moto-Hakone Bay, blending sacred prayer with breathtaking spectacle—an unforgettable summer night unique to Hakone.

Ceremonial Flow of Kosui-sai

Held on the eve of Hakone Shrine’s grand annual festival, Kosui-sai unfolds over three solemn and awe-inspiring parts:

1. Ritual on the Shrine Grounds (18:00 / Hakone Shrine Grounds)

The evening begins within the serene precincts of Hakone Shrine. Priests offer red rice (sekihan) to Kuzuryu Okami, followed by sacred purification, lighting of ritual lanterns, and prayers. This solemn ritual sets the spiritual tone for what follows on the lake.
 

2. Ritual on the Lake (19:00 / Lake Ashi)

A ceremonial boat carrying the chief priest sets off toward the heart of Lake Ashi, accompanied by music and attendants’ vessels. There, the priest offers precisely 3 shō, 3 gō, and 3 shaku (about 5.4 liters) of red rice to Kuzuryu Okami. This sacred ritual, passed down through generations, is known only to the chief priest—its exact location and timing remain a secret. The lake becomes a stage for a mysterious and deeply spiritual act, rarely seen and never forgotten.
 

3. Kosui-sai Fireworks Display (20:00 / Approx. 5,000 Fireworks)

As the offering ritual concludes, the night sky bursts into color. About 5,000 fireworks are launched from Moto-Hakone Bay, including underwater fireworks and floating lanterns. Lake Ashi glows with reverence and celebration, wrapping the night in light and prayer.

A Sacred Secret Known Only to the Priest

Kosui-sai marks the beginning of Hakone's week-long Summer Festival around Lake Ashi. Its roots lie in an ancient legend: a poisonous nine-headed dragon once terrorized the people until the monk Mangan subdued it through three days and nights of prayer, transforming it into Kuzuryu Okami, the lake’s revered deity.

To this day, the same amount of red rice—3 shō, 3 gō, and 3 shaku—is placed on the ceremonial boat and offered into the lake. The exact offering spot, known as the lake’s sacred center, is passed down only to the chief priest. It is said that the red rice has never once resurfaced.

This secretive and sacred ritual continues to be quietly upheld, connecting the present to a mystical past, and infusing Hakone’s summer nights with reverence, beauty, and timeless mystery.
 

Date: July 31 every year (on the eve of the Hakone Shrine Grand Festival)

Location:Hakone Shrine and the surrounding area of Lake Ashi / Moto-Hakone Bay

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