本文乃就1872至1873年間日本外務卿副島種臣利用琉球人在臺灣南部「番地」遭殺害一事，提出「番地無主論」主張的形成，乃至運用等一連串過程所進行的實證考察；同時也對過程中重要相關議題，例如，美國的影響、副島身分、軍艦派遣，及「番地」問題交涉遲延等，進行再檢討。其結果大致可歸納如下：1.副島的臺灣「番地」認知，最初是基於外務省的臺灣資料調查，認為「番地」非清國所有，最後在5號意見書中，從國際法角度明確主張「番地」乃「無主之地」的「番地無主論」。2.美國方面對於副島的影響主要表現在，由直接對臺灣「番地」出兵改為對清外交交涉，及激發其占領「番地」的企圖心。3.從副島對「番地無主論」的堅持，可以看出他對於占領臺灣「番地」所具有的強烈企圖心與對外擴張性格。4.特命全權大使的任命、軍艦的派遣等，皆有其外交慣例及實務上的需要，不能僅以政府與副島間的對抗關係來說明。5.避免琉球問題的爭議與「番地無主論」是副島在北京交涉時的兩大策略。藉由上述檢討結果，除了更清楚、完整地闡明副島的對外擴張性格外，同時也進一步釐清相關的重要政治、外交議題，為牡丹社事件研究提供新的視角與觀點。

This article discusses the transition from the formation to the application of the statement on “Terra nullius in Taiwanese aboriginal area,” proposed by Soejima Taneomi from 1872 to 1873, as a response to the massacre of Ryūkyūan residents in “Taiwanese aboriginal area.” Coherently, this article also reinspects several significant factors and subjects during this process, such as the influence of the United States, Soejima’s status, the dispatch of warships, and the procrastination on the political negotiation in regard to “Taiwanese aboriginal area.” There are some generalizations as follows: First, Initially, Soejima considered that “Taiwanese aboriginal area” didn’t belong to the Qing Dynasty according to the Ministry of Foreign Affairs of Japan’s report on Taiwan. Afterwards, in the Fifth Opinion, he made an explicit statement on “Terra Nullius in Taiwanese Aboriginal Area” from the standpoint of international law. Second, The United States’ major influence on Soejima was converting his action from dispatching troops to seeking diplomatic negotiation with the Qing Dynasty, and after that inspiring his ambition to occupy the “aboriginal area.” Third, Based on Soejima’s persistence in his assertion of “Terra Nullius in Taiwanese Aboriginal Area,” it was obvious that he had strong intentions on expanding the border of Japan and had taken aim at “Taiwanese aboriginal area.” Fourth, The appointment of ambassadors and naval dispatches were usual diplomatic practices with actual demand and wouldn’t be relating them directly to the confrontation between Soejima and the Government of Japan. Fifth, Avoiding the controversy over the Ryukyu incident and the statement on “Terra Nullius in Taiwanese Aboriginal Area” were Soejima’s main strategies for diplomatic negotiation with the Qing Dynasty.