PACE publishes an annual guide to the state of publishing in Japan, with extra information about getting translation rights and literary agencies that you can hire to help you with the process.
In general, copyright protections extend 50 years beyond the death of the author in Japan and 70 years beyond the death of the author in the United States. Copyright protections may be extended or sold as well.
You would think that information about books would be easy, but it isn't. There used to be an annual publications called 著作権台帳 published in print. It has ceased publication. Now you need to check the author's dates in NDL OPAC, the name listed on the colophon, and check with publishers.
This is the website to consult in either English or Japanese for the laws, case studies, and general Q&A about copyright concerns. See also the links to organizations that protect the rights for different kinds of materials (music, dramas, literary)