The Kanji_Test.tex file you attached is encoded in UTF-16. I'm surprised you got output of any kind using the file as is.
However, if you're sure the file is UTF-8 on your end, try the
following, maybe inputenc is the culprit:
\documentclass{article}
\usepackage{fontspec}
\begin{document}
\fontspec{IPAMincho}住民基本台帳ネットワーク
\end{document}
Hope this helps,
Andy
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