National Diet Library (NDL) = 国立国会図書館学位論文
As of December 2019, includes 204, 760 dissertations. Among the dissertations received by the NDL between 1991-2000, the NDL digitized the ones whose author granted the permission. Also contains the doctoral dissertations in digital format for which degrees were conferred in or after April 2013.
Also refer to 「国内博士論文」調べ方案内
in the NDL's Research Navi for further details about using the database and more.