We investigated bird nest predation on Miyake Island where the Japanese Weasel Mustela itatsi has been introduced. From observations of Japanese Bush Warbler nests, we estimated survival rates of 0.498 and 0.848 in incubation and nestling stages, respectively. Artificial nests coupled to an automatic image recording system showed that weasels were bird nest predators on three occasions. No other potential predators visited the nests; before the weasel introduction, nest predation was rarely reported on the island. The incidence of predation on artificial nests increased as a function of the density of predated artificial nests in the neighborhood. Other nest-site characteristics were not related to nest predation, except for height above the ground. Lower nests had slightly higher risks of predation. Hence, the fate of a nest appears to depend on whether it is located within home ranges of the Japanese Weasel. Weasels may easily find nests located within their activity ranges irrespective of their site characteristics. Japanese Weasels must be regarded as a major threat to breeding birds on Miyake Island.