indoor hot-spring bath
Refers to a bath inside a building that uses natural hot-spring water, as opposed to an outdoor bath (露天風呂).
This inn has both an indoor hot-spring bath and an outdoor bath.
Standard kanji spelling.
Mixed kanji-kana spelling; less common but still used.
Compound of 内 (uchi, 'inside') and 湯 (yu, 'hot water').