The sweet potato weevil, Cylas formicarius (Fabricius), is a destructive insect pest of the sweet potato, Ipomoea batatas (L.) Lam. We investigated the geographical distribution and seasonal abundance of C. formicarius on Kikai Island using traps baited with synthetic sex pheromone (hereafter, pheromone trap). Male weevils were captured in most parts of Kikai Island using pheromone traps. In residential areas with a high density of houses, the numbers of males captured using pheromone traps were fewer than those of males captured in other areas. These results may be explained by the fact that residential areas offer a smaller area for the growth of the wild host plants. The seasonal abundance of the weevil population in Kikai Island was estimated based on the number of males captured using pheromone traps. The number of captured males increased from July or August and reached its peak in late September to October. Subsequently, it decreased rapidly and remained low from December until the next June. These results showed that the weevil population size on Kikai Island was small during winter, began to increase gradually in spring, and was the largest in September or October, reaching a peak population density in the period of 1 year.