A Will is a written statement of what a person wants done with their property after they die.
http://www.nycourts.gov/courthelp/WhenSomeoneDies/will.shtml
When a foreign resident wants to make a Will, what laws are applied?
Act on General Rules for Application of Laws (法の適用に関する通則法:Rule1) is a basic law to think about which laws are applied.
Regarding Wills, there are other two important rules:
1. Convention on the conflicts of laws relating to the form of testamentary dispositions(遺言の方資金関する法律の抵触に関する条約：Rule2)
https://www.hcch.net/en/instruments/conventions/full-text/?cid=40
2. Act on the Law Applicable to the Form of Wills(遺言の方式の準拠法に関する法律：Rule3)
http://www.japaneselawtranslation.go.jp/law/detail/?id=2000&vm=04&re=01
When a foreign resident passed away, the national law is applied. Japanese law is not applied (Rule 1, Article36).
But as of a Will, the Will, which follows Japanese law, is valid (Rule2 and Rule 3).
Recently many foreign people live and have property in Japan. To avoid complicated inheritance procedures, making a will may be an effective solution.