Black-billed Magpies (Pica pica) have been increasing and expanding rapidly in Hokkaido since the introduction in 1980s (Horimoto 2004). It is afraid that magpies adversely influence on a native ecosystem through the predation of songbirds or competition with closely-related species. Nest-site selection, one of important factors to determine breeding success of birds, may have caused rapid increase and expansion of magpies in Hokkaido. In this study, we investigated the nest-site selection of magpies in Tomakomai, from two aspects, nesting substrates and surrounding environments. We took data at nest sites and contrast sites set at 100m north of each nest, and compared parameters between them. Magpies preferred sites where tree density was high when nesting on trees, while tree density did not affect when nesting on human-made structures. The heights of nests and nesting substrates were higher when nesting on human-made structures than when nesting on trees. Moreover, magpies preferred higher trees than those nearby. These results suggest that magpies prefer nest sites to avoid nest predators. We suggest that nesting on high position enables magpies to watch over avian predators and prevents mammalian predators from reaching nests.