Kusatsu Onsen, high in the Gunma mountains at 1,200m altitude, has been ranked Japan’s most popular hot spring resort (according to Nihon Onsen Hyakusen rankings) for over 30 consecutive years. The town centers on the Yubatake (hot water field) — a large wooden structure channeling naturally flowing 55–95°C spring water through cooling chutes before it is distributed to the town’s baths. The sight of steam rising from the Yubatake at night, illuminated against dark mountain air, is one of Japan’s most iconic onsen images.
The Yubatake
The Yubatake is the visual heart of Kusatsu — a series of wooden slotted trays (yubatake, literally “hot water field”) occupying the town center through which 32,300 liters of 55°C spring water flows every minute. The trays simultaneously cool the water to bathing temperature and allow minerals to crystallize and deposit on the wood surfaces. Surrounding the Yubatake are foot baths (free, always open), souvenir shops, and the illuminated nighttime spectacle that draws visitors regardless of whether they plan to bathe. The spring water is highly acidic (pH 2.1 in some sources) — among the most acidic natural spring water in Japan, famous for its bactericidal properties.
Yumomi: The Cooling Ritual
Yumomi is Kusatsu’s traditional hot spring cooling technique — a group of women in traditional dress stir large paddle-like boards (yumomi-ita) through a bath of hot spring water to cool it to bathing temperature without diluting the mineral content. Traditionally a communal activity with accompanying folk songs (yumomi uta), it is performed as a living cultural demonstration at the Netsunoyu public bath near the Yubatake several times daily. Visitors can purchase a short participation session where they take a paddle and stir the water themselves alongside demonstrators.
Bathing in Kusatsu
Kusatsu’s highly acidic water (pH 2.1) is powerful — even a 3-minute bath can feel intense, and the town’s traditional bathing method (jikan-yu, “timed bathing”) involves immersing for exactly 3 minutes in very hot water multiple times per session. The public Otaki-no-yu and Goza-no-yu bathhouses (¥600–¥800) are open to all visitors. Sai-no-kawara Rotenburo is a large outdoor public bath in a park setting, open for a small fee. Ryokan guests typically have access to both private and shared baths with the property’s directly piped spring water.
Kusatsu Surroundings
At 1,200m altitude, Kusatsu sits on the slopes of Shirane-san volcano in the Joshinetsu Kogen National Park. The plateau above the town offers hiking in summer and skiing in winter (Kusatsu International Ski Resort is a short bus ride from the onsen area, allowing a combined ski-onsen day). Autumn foliage in October–November transforms the surrounding mountain forest into intense reds and yellows framing the steaming Yubatake. The volcanic crater lake Yugama on Shirane-san is a 90-minute hike from the ski area — an emerald-green acid lake of extraordinary color.
Practical Tips
- Getting there: JR Agatsuma Line from Naganohara-Kusatsuguchi Station then bus (25 min); or direct express bus from Shinjuku (Tokyo) — 4 hours, ¥3,200. The JR approach is scenic but slow; the bus is faster
- Skin sensitivity: Kusatsu’s highly acidic water can irritate sensitive skin or recent cuts; limit initial soak times and rinse thoroughly after bathing
- Evening Yubatake: The Yubatake is illuminated after sunset — evening visits are less crowded and more atmospheric than daytime
- Ryokan rates: Mid-range ryokan with two meals run ¥15,000–¥25,000 per person; budget guesthouses from ¥8,000 per person without meals
- Ski-onsen combination: December–March allows morning skiing, afternoon soaking — a combination that explains Kusatsu’s year-round popularity