Shōdo-shima was famous for its soy beans long before olives arrived, and several old soy-sauce companies are still in business here. Marukin has a small museum with displays of the sauce-making process, old implements, photos and interesting facts you never knew about the ubiquitous brown stuff. There are good English explanations, and you can try the surprisingly tasty soy-sauce-flavoured ice cream.
It's on the main road between Kusakabe and Sakate.
