Loaded with major ions — the essential minerals (Ca, Mg).
Buoyancy
Standard buoyancy
LowHigh
Soft water — natural, classic buoyancy.
BicarbonatesSodiumCalcium
Koga Onsen is a relaxing hot spring facility in Koka city (historically romanized as Koga), Shiga Prefecture — a region world-famous as the birthplace of the Koka school of ninjutsu, one of the two great ninja traditions of feudal Japan. The bicarbonate thermal waters provide gentle warmth and skin-softening benefits, offering a civilized counterpoint to the area's martial heritage celebrated at the Koka Ninja Village and historical museum. The rolling Shigaraki Plateau landscapes of cedar forests, ceramic kilns, and bamboo groves surrounding the onsen add a uniquely artisanal and atmospheric dimension to the bathing experience.
Insider tip — Koga Onsen sits in the broad Kanto Plain of Saitama, where deep drilling reaches dark, peat-rich bicarbonate waters typical of the region. The black-tinted water, locally called kuroyu, softens skin and is associated with beauty benefits. Day-spa facilities here attract city dwellers escaping the Tokyo metropolitan area for weekend relaxation. The flat farmland surroundings give the experience an unassuming, genuinely local feel.
