I haven't seen the manga version, but loved the movie adaptation with Abe Hiroshi in it.

@strobe999 linked to the Wiki article for the manga. The story follows Lucius, an architect in ancient Rome, who timeslips into modern day Japan. He carries ideas from modern Japan's bathing culture back and builds the public baths of Rome. It's a comedy, but a very informative one at that.

I saw that show, and as an onsen fan loved it, but recently I stumbled upon a pretty interesting article on how Roman baths were... not all that great from a health/hygiene perspective: https://archive.is/Y1un4