This paper sheds light on a relatively unexplored component of Japan's security architecture: the national security secrecy system. Among the protection measures underpinning this system, particular attention is paid to the legal aspects. This system has been caught between two competing pressures. On the one hand, the United States and domestic conservative forces have consistently sought to strengthen what they consider to be an unsatisfactory system of safeguarding national security secrets in Japan. On the other hand, left-leaning political actors, the media and public opinion, informed by the norm of antimilitarism—understood as a deep distrust of Japan's military establishment and opposition to its foreign deployment, arid also broadly predicated on fear of democratic retreat—have been able to thwart these attempts. As a result, Japan lacks a specific anti-espionage or state secrets law and has earned the reputation of a "spy heaven" where foreign agents, domestic collaborators and information leakers have been able to act with seeming impunity. However, over the past decade, rising bilateralism in the security and intelligence fields, under which Japan acts to consolidate alliance relations with its superpower patron, facilitated by the rise of regional security threats and a shift in the domestic political terrain, have contributed to an incremental erosion of the antimilitaristic constraints on the legal component of Japan's security secrecy system. Japanese governments have moved to bolster the secrecy protection regime, although reforms are not as significant as what they would be absent residual antimilitarism. [ABSTRACT FROM AUTHOR]