Bagworm has been well known to people as the symbol of winter in Japan, and it used to be found at everywhere. Japanese children sometimes found it when they were hanging at and they might play with it or observe it. However, we get decreasing the chance to see then not only causing becoming an adult but also the number of them is decreasing desperately especially, Japanese Big Bagworm. Because the parasite fly have been identified in Kyushu area. That implies they came from continent that must be China, and it decrease Bagworm radically and let them be disappearing. This insect parasitizes only Japanese Big Bagworm. However this must not be cause of extinction. They have abnormally power of spreading and it causes the decreasing Bagworm. Usually, an invaded creature increases suddenly, because they have no natural enemy. Considering that, their increasing is abnormal. Parasite creatures usually try not to increase too much. Because loosing their host means their extinction. However, this parasite fly parasitized 90% of their host and it was too high percentage for any parasite creatures. Some prefectures in west Japan appointed Japanese Big bagworm as an endangered species and started to protect them. Eco system also works like trying to keep the variety of species. The parasite bee that parasitizes this parasite fly was founded. This natural enemy must prevent the extinction from Bagworm. The study shows that Japanese Big Bagworm is still being, however we also keep trying to protect them. We have to avoid to lose this traditional insect.