Several decades have passed since the first call was made toward political scientists, among other social scientists, to devote attention to human rights and enrich, through empirical investigations, our understanding of how human rights implementation works. Today, the political science discipline is finally beginning to respond to this appeal, with an increasing number of scholars making it their goal to isolate variables that obstruct or facilitate human rights implementation. The current paper joins this burgeoning body of literature by offering a within-case analysis of Japanese prison policy-making. In particular, I compare the 2005 bill updating the Prison Law (Kangoku hō) that had been in force in Japan since the Meiji era with an earlier draft version of that bill which appeared in parliament on three occasions since the early 1980s. On the basis of the convergence of these bills in terms of seeking to align Japan with the evolved new global standards for convicted prisoners' treatment, I argue that a three-decade delay occurred in the implementation of prisoners' rights in Japan. To account then for this delay, I point to a provision pertaining to the criminal procedure (i.e., pre-conviction) which was incorporated in the law in question in 1908 merely for pragmatic reasons, and with regard to which the modern-time stakeholders of the bar and the police could not find agreement. Ultimately, the message that this case deals to the political science of human rights is that institutional–infrastructural factors, such as ties of legal nature, matter to human rights implementation.