In Japan, the propagation of sweet potato is practiced mainly by planting sprouts, but the planting of small tubers method has been unnoticed by the majority though it seemed to be useful. The small seed tuber planted in the ground grow itself but it can not become a fleshy root of good quality. Therefore this method is not yet utilized except for forage to domestic animals and fowls in this country. The writers had improved this method by exposing setting-tuber to light immediately after its sprouting. The growth of setting-tuber which had been exposed to the light is inhibited to thicken and the new fleshy roots produced from the original setting-tuber are promoted to grown this study, the growing process of sweet potato plants grown from setting-tuber by the exposing method was traced, comparing with that of plants grown from setting-tuber which had been kept in the ground. Dry weight of setting-tubers were gradually decreased till its sprouting time and the tubers kept in the ground grew more rapidly than that of exposed tuber, but as to the growth of new fleshy roots, fleshiness was lower in the former case than the latter, and the growth of tops was almost similar in both cases. Differences in anatomical picture of setting-tubers in both cases were traced. In the light exposed plot; Cell membrane of epidermis was thicker at the harvest time than that of setting time, and there were scarcely new cells. About 30 days after setting, the appearance of chlorophyll was recognized in the epidermis and cortex cells, and the chlorophyll was observed also in parenchymatous cells diversed from cortex. And suberized cells were seen about 50 days after tuber setted. Arrangement of xylem became like a belt, and vessels were aggregated densely in this belt. Degree of lignification of the cells around vessels was remarkable, and the lignificated cells were closed together about 80 days after setting. In nonexposed plot; About 50 days after tuber setted, the surface of setting-tuber was cracked and epidermis was formed on this crack surface and new cells developed from the old tissues as chimera. Chlorophyll was not seen in the cortex and number of suberized cells were less than that in exposed plot. Layer of cortex was narrow and the size of cells was not uniform. Development of xylem was remarkable and the arrangement of xylem became as a zigzag ring. Vessels were dispersed and the degree of lignification of the cells around vessels was less than that of exposed plot. The facts that the transformation in tissues and appearance of chlorophyll in the setting tuber of sweet potato induced by exposing to light, as observed in this study, may be considered to be related to the inhibited growth of setting-tuber growing.