Short answer. Under Japan's Water Supply Act, a water utility that learns its water may harm health must stop supply immediately and warn users that the water is dangerous to use 1. Utilities must also give customers their water-quality test results 1. Store 3 litres of water per person per day for drinking and cooking, for 3 days; the Prime Minister's Office says 1 week is desirable for large-scale disasters 27.
How do I know if my tap water is unsafe?
The legal duty sits with your water utility (水道事業者), usually run by your municipality. Article 23 of the Water Supply Act says that when a utility learns that the water it supplies may harm human health, it must stop supply at once and take steps to let the people concerned know that using the water is dangerous 1. Article 24-2 requires utilities to give customers the results of their water-quality tests and other information about the service 1.
Phaenyx found no single national list of boil-water or do-not-drink notices for Japan. Check your own utility's or municipality's website and follow its instructions. For what each kind of notice means at home, read What to do during a boil-water notice and Do-not-drink notices. See our Japan alerts page for how warnings reach your phone.
How much water should I store?
MAFF's figure is 3 litres per person per day, for drinking and cooking: "飲料水と調理用水として1人当たり1日3リットルが必要です" 2. The Prime Minister's Office sets drinking water at 3 days' worth, at 3 litres per person per day, and says a 1-week stock is desirable for large-scale disasters 7. MAFF notes that restoring lifelines has often taken a week or more after past disasters 2. Tap water kept in a clean container filled to the brim, capped and stored away from direct sunlight in a cool room can be used as drinking water for about 3 days 2. You also need separate water for flushing toilets: the Prime Minister's Office suggests keeping a plastic tank filled with tap water or leaving the bath filled 7.
Full figures are on our Japan emergency supplies page. For containers and rotation, see Water storage containers and How much water to store per person per day?.
Is Japan short of water?
Not on average, but dry summers still bring restrictions. In WRI's Aqueduct 4.0 model, the population-weighted median of Japanese cities sits in the "low to medium" category for current water stress and stays "low to medium" in the 2050 business-as-usual projection; drought risk is "medium" 5. These are city medians: a given town or river basin can sit higher or lower.
The national drought portal of the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) reports current conditions by region 3. Its update of 27 October 2026 lists drought headquarters opened at six regional development bureaus since early August 2026, three of which (Kantō, Hokuriku, Kinki) have since closed 3. Restrictions on household water supply (給水制限) during droughts are listed on MLIT's water-supply disaster and drought page 4. For household steps, see Drought preparedness and No running water.
What if I have a private well?
National guidelines for drinking wells (飲用井戸等衛生対策要領) call for testing at least once a year; for a well serving only the owner's own home, yearly testing is described as desirable 6. If the water may harm health, stop supplying it, tell the users and contact the public health centre (保健所) for instructions 6. See Private well emergencies.
What should I do after a flood?
Follow your water utility's instructions, and see Water safety after a flood and Water purification. A household plan can keep your water stock, alert settings and utility contact in one place.
Frequently asked questions
Who warns me if tap water is unsafe in Japan?
Your water utility. The Water Supply Act requires it to stop supply and warn the people concerned when the water may harm health 1.
How much water does Japan recommend per person?
3 litres per person per day for drinking and cooking 27. Keep at least 3 days' worth; 1 week is desirable for large-scale disasters 7.
Where can I check drought restrictions?
On MLIT's national drought portal and its water-supply drought page 34.
Sources
- e-Gov 法令検索 (Digital Agency, Government of Japan) — 水道法 (Water Supply Act, Act No. 177 of 1957), Articles 23 and 24-2 — https://laws.e-gov.go.jp/law/332AC0000000177 (accessed 2026-10-27)
- 農林水産省 (MAFF) — 知って備える 家庭備蓄のイロハ（aff 2026年3月号 特集） — https://www.maff.go.jp/j/pr/aff/2603/spe1_01.html (accessed 2026-10-27)
- 国土交通省 (MLIT), 水管理・国土保全局 水資源部 — 渇水情報総合ポータル — https://www.mlit.go.jp/mizukokudo/mizukokudo_mizsei_kassui_portal.html (accessed 2026-10-27)
- 国土交通省 (MLIT) — 上下水道：災害・渇水情報 — https://www.mlit.go.jp/mizukokudo/watersupply/stf_seisakunitsuite_bunya_0000096246_00003.html (accessed 2026-10-27)
- World Resources Institute (WRI) — Aqueduct 4.0 (water stress, 2050 projection and drought risk) — https://www.wri.org/aqueduct (accessed 2026-10-27)
- 国土交通省 (MLIT) — 飲用井戸等衛生対策要領 — https://www.mlit.go.jp/mizukokudo/watersupply/content/001979425.pdf (accessed 2026-10-27)
- 首相官邸 (Prime Minister's Office) — 災害が起きる前にできること — https://www.kantei.go.jp/jp/headline/bousai/sonae.html (accessed 2026-10-27)
Turn this into action
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Sources
- e-Gov 法令検索 (Digital Agency, Government of Japan) · 水道法 (Water Supply Act, Act No. 177 of 1957), Articles 23 and 24-2 checked 2026-10-27
- 農林水産省 (MAFF) · 知って備える 家庭備蓄のイロハ（aff 2026年3月号 特集） checked 2026-10-27
- 国土交通省 (MLIT), 水管理・国土保全局 水資源部 · 渇水情報総合ポータル checked 2026-10-27
- 国土交通省 (MLIT) · 上下水道：災害・渇水情報 checked 2026-10-27
- World Resources Institute (WRI) · Aqueduct 4.0 (water stress, 2050 projection and drought risk) checked 2026-10-27
- 国土交通省 (MLIT) · 飲用井戸等衛生対策要領 checked 2026-10-27
- 首相官邸 (Prime Minister's Office) · 災害が起きる前にできること checked 2026-10-27