The website of the Ministry of Health, Labor and Welfare shows what kind of quarantine measures will be taken when entering Japan.
After June 1st, if you enter Japan, you will be able to know what the quarantine measures will be when you enter Japan, depending on the classification of the country / region where you were staying and the vaccination status.
If you are planning to enter Japan from overseas, or if you are going abroad soon, why not try it as a reference?
The corresponding page of the Ministry of Health, Labor and Welfare is as follows.
https://www.mhlw.go.jp/stf/covid-19/bordercontrol.html