Mt Fuji is the most famous volcanoes in Japan. Mt. Fuji has no rivers and its land surface has been strongly disturbed by eruption. How does the difference of eruption histories affect ecology on Mt. Fuji? To reveal the effect of volcano on avifauna, we compared breeding avifauna and insect abundance between marginal two type forests which have significantly different eruption histories. We censused breeding bird species and determined their guilds. We captured insects with malaise traps and beating. In the young pine forest on larva substrate, there were abundant spiders Araneae and locust Orthoptera . Bird species which eat spiders bred in the young pine forest. Another old broad leaf forest had many insect ( Lepidoptera , Hymenoptera , Diptera , Coleoptera ) and many breeding bird species without spider eaters. We found ground nesting bird species never breed on larva structures. In summary, the breeding avifauna was strongly affected by eruption because nest site availability for ground nesters and insect abundance especially spiders are different.