It is essential to investigate the cultural exchange with surrounding areas in analizing the formation and developing process of Japanese prehistoric agriculture. The study on the 'Southern Route' is especially important because it is assumed that the 'Southern Route' had influenced deeply on the formation of farming culture of vegetative root crops which had been the staple crops in the Japanese fundamendal agriculture. In general speaking, the 'Southern Route' consists of the continental route and the islands route. I have mainly studied the islands route, especially the route of the Kuroshio Current, which has Nansei Islands, Formosa, the Philippines along the Kuroshio Current. Comparing similarities of the traditional farming culture, language, rituals, archaeological ruins and remains between the regions along the Kuroshio Current, we can find spatial continuity and discontinuity of the similarities. This discontinuity, however, is not the absolute discontinuity, but is a partial one, and it is considered to be a phenomenon appeared on the outer layer of culture when you see it in time series. Therefore, it is important to investigate what really exists in the depth of such discontinuity, or to find out why it looks discontinued on the outer layer. This paper, as a process of investigating such depth under discontinuity, studied Lanyu (Botel Tobago), the small island in the southeast of Formosa, and recorded and investigated the farming styles, land ownership, and land usage, based on the field study of the farming system of root crops in Lanyu. Then, we compare them with those of Japanese Nansei Islands, Northern Philippines, and Fiji. As the result, Fiji and Nansei Islands showed the distinct contrast. Fiji people have developed the most rich and active farming culture of root crops, but Nansei Islanders have grown a few varieties with dedicated care. The root crops farming culture of Lanyu and Northern Philippines, which basically position between Fiji and Nansei Islands, showed similarity of something in-between concerning their characteristics. With close look, we found less variety in Lanyu which resembles the Nansei Islands, and ample variety in Northern Philippines which resemble Fiji. These positioning of similarity agrees with the idea that Lanyu belongs to the cultural diffusion area of the farming culture of vegetative root crops, following the Kuroshio Current from the Philippines, Formosa, and to the Nansei Islands.