Japan is a traditional great oceanic nation and sea power that attaches great importance to oceanic issues. In recent years，with the expansion of its jurisdictional range of sea area and increasingly protruding of disputes with neighboring countries on oceanic rights and interests，Japan has been constantly enacting new marine policy，marine strategy and law of the sea，thus actively promoting its established comprehensive “oceanic state founding” strategy of transforming from an “island country” to a “sea country”. All sorts of steps taken by Japan in the marine sphere will bring about major challenge and influence to the maritime security situation of Northeast Asian sea area，especially of neighboring China.