The most accurate source of information isn't the ryokan's marketing copy — it's the notice posted in the changing room. Knowing how to read it sharpens your judgment when choosing a bath.
See Hakone private-onsen ryokan →What the disclosure requirement covers
Under a May 24, 2005 revision to the Hot Spring Act enforcement regulations, facilities must post — in addition to the traditional mineral-content and health-caution information — whether the water is diluted, reheated, recirculated/filtered, or treated with bath additives/disinfectant, along with the reason for each, in a visible spot inside the facility. The rule followed cases of facilities that diluted, reheated, and recirculated water while still advertising it as “100% natural hot spring.”
What each of the four items means
- Dilution: cold water added to the source water. Usually disclosed as temperature adjustment when the source is too hot.
- Reheating: heating the water to raise its temperature. Usually disclosed for a source that's too cool, or to maintain temperature.
- Recirculation (filtering): filtering and reusing the water. Usually disclosed for hygiene management or to maintain water volume.
- Bath additives / disinfection: use of bath additives or disinfectants such as chlorine. Disinfection for hygiene reasons is common and not unusual.
What else is on the notice
The notice also lists the source name, mineral type, source temperature, the results and date of the mineral analysis, health cautions (conditions that should avoid the bath), and bathing precautions. Because the mineral analysis must come from a registered testing body's latest results, it's a solid primary source if you want to confirm the mineral type (see hot spring terms explained).
How this relates to “free-flowing” claims
Free-flowing hot spring water (gensen kakenagashi) generally means the water isn't recirculated and is used once, but even free-flowing water can involve dilution or reheating for temperature control. Japan's National Consumer Affairs Center has logged complaints of “the label says free-flowing, but the water isn't overflowing the tub” — rather than judging by the label alone, checking the four disclosed items on the notice is the reliable approach (see ryokan with free-flowing hot spring water).
Putting this to use when choosing a ryokan
Before booking, check the ryokan's official site or plan details for how the water is treated (free-flowing vs. recirculated, dilution, reheating). Once you check in, confirm what you read against the notice in the changing room. If water freshness is your top priority, see ryokan with free-flowing hot spring water; if you just want to know whether it's a genuine hot spring at all, see how to tell hot spring water from reheated water.
FAQ
If the water is diluted, does that mean it isn't a real hot spring?
Not necessarily. Dilution — often done to bring down the temperature of source water that's too hot — has a disclosed reason, and it doesn't change the fact that it's still hot spring water.
Where is the notice posted?
Facilities are required to post it somewhere visible inside the building, typically the changing room. If you can't find it, ask at the front desk.
Last updated: 2026-07-17 ・ Reservations are on Rakuten Travel (Japanese site).