1.Introduction 2.Change of Koizumi's words and deeds by the prewar situation 3.Koizumi's change to anti-American argument 4.Koizumi's anti-American argument 5.Conclusion This paper makes clear that even Sinzo Koizumi converted his pro-American theory into violent anti-American argument at the end of the Pacific War, by investigating the changing process of Koizumi's opinion in the period concerned.