Sweet potato is a plant, a root vegetable in English speaking countries but sweet potato is a baked confection in Japan, using sweet potato. Then, the confection is not familiar outside of Japan, which means you have to try it during your stay in Japan.
To make the confection, sweet potato must be strained and mixed with sugar, milk, butter and other seasonings and baked as oval shape on foil or cooking paper.
On the other hand, the plant, sweet potato is called satsumaimo; literally Satsuma's potato. Satsuma was a province in southern Kyushu which is currently a part of Kagoshima prefecture. The plant was brought to Satsuma province from China via Okinawa. After harvesting at Satsuma province, the plant was widely spread to other parts of Japan.