From the National Library of Japan:
The bibliographic data of the National Diet Library (not including body text) became available for anyone at no charge, whether the purpose of use is commercial or non-commercial, from April 1, 2019.
The bibliographic data includes authority data (data of authors and keywords which are arranged for use as clues when searching materials), data of periodicals index, and information of table of contents, as well as bibliographic data (titles, authors, publishers, and date of publication of materials) made by the National Diet Library.
In addition, data provided by the National Diet Library, which the National Diet Library obtains by links to external organizations, is also available without application, as long as the National Diet Library is permitted to provide the data in accordance with the contract with the organization.
Services which currently provide bibliographic data are as follows. Please check the introduction and help pages of each service when using that data. Please take this chance to try to use the bibliographic data of the National Diet Library in various situations.