There are few universities specializing in pure Japanese, and private ones are: Kyoto Foreign Studies University, Kyoto Institute of Technology, Minghai University, Nagoya Foreign Studies University, Mubai University, Lize University, Dadong Culture University and specialized universities.

State-owned: Osaka University, tokyo university of foreign studies.

What I checked for you is that there is a Japanese major in the department. If you want to take the postgraduate exam, you need to go to official website of these schools to see if there is a Japanese major.