Viola raddeana is a perennial wetland species listed as endangered species (Category II: vulnerable) in the Japanese Red List. We examined yearly changes of its density at the grassland maintained by prescribed burning. We also conducted a study site of 5 year interruption of burning. Interruption of the prescribed burning resulted in rapid reductions in the density of V. raddeana, especially that of seedlings. However, the density of V. raddeana decreased under annual prescribed burning. Furthermore, increase in the rate of large sized over-wintered individuals was observed especially in the interrupted site. It is expected that a large individual has the large seed quantity of production. These results indicate that conservation of V. raddeana requires promotion of not only seedling recruitment by prescribed burning which promote improvement of environment such like light condition, but also the maintenance of populations which include large individuals capable of producing seeds. Hence, fire-free intervals may be required.