‘Deep-free trade agreements (FTA)-maker-multilateralist’ nations are obliged to find unconventional ways to co-exist with China in countries along the Belt and Road Initiative (BRI); and this obligation is becoming more obvious. There is a possibility for Japan’s involvement in enlisting soft law mechanisms in order to ensure China’s BRI projects conform to high standards; and this serves as a main argument of this article. In my view, with its unique geographic position, the existing fierce rivalry in the Association of Southeast Asian Nations, and the country’s intertwining market with China, Japan’s capacity to reform the BRI from the inside should not be underestimated. On the contrary, by persisting with high Quality Infrastructure Investment (QII) principles promoted in the last three years, Japan has been successful in nudging China toward these principles. The country has also enabled China to endorse the notion of high-quality infrastructure at the second BRI forum. With Japan’s leadership, the G20 Osaka Summit has contributed considerably into bolstering of the QII principles. Moreover, despite being less institutionalized and having soft law nature, the Japan-China Memorandum on Business Cooperation in Third Countries is effective: The Memorandum utilizes and suppresses the harm caused by the economic activities of Chinese businesses in third-country markets. Presently, Sino-Japanese collaboration in Thailand’s East- West Economic Corridor Program is testing whether the Chinese government is capable of earnestly engaging in ‘Third Party Market Cooperation’, or whether this endeavour is futile.