Sprouting from the root-tuber of growing plant have been known in some varieties of sweet potato. Using the four cultivars, Chugoku-25, Hayato, Kokei-14 and Norin-1, which had difference in the sprouting habit, experiments were carried out to get knowledge of the regulating factor/s on the development and the sprouting of the adventitious bud in sweet potato. (1) No varietal difference was observed microscopically on the developmental pattern of adventitious bud until the 60th day after planting. (2) In later day, some of the adventitious buds in the two varieties, Chugoku-25 and Hayato had grown in their length and had sprouted. On the other hand, most of the adventitious buds of the other two varieties, Kokei-14 and Norin-1 never elongated until havest time. (3) The varietal difference on the sprouting habit, therefore, may be based on the difference among the varieties on the elongation of adventitious bud, but not on the developmental pattern of them. (4) By means of grafting which was undertaken among the four varieties, it was revealed that the elongation of adventitious bud was influenced by the characteristics of a variety used as a scion. And it was supposed that the substance/s transfered from scion to stock (from shoot to root-tuber) inhibited the growth of sprout. (5) On the other hand, the influence on the elongation of adventitious bud could be observed also in the characteristics of a variety used as a stock. Therefore, it is concluded that the sprouting of the adventitious bud in root-tuber may be regulated by the substance/s transfered from shoot and also by the endogenous factor/s in root-tuber.