[Sangetsu]

The general rule is that you can apply for PR after 5 years of living in Japan. But there are some caveats. If you live in Japan for 4 years on a work visa, and then get married, you have to wait 5 years from the change in status of your visa, meaning it would take 9 years.

And, there is no rule requiring Japan to give you PR after 5 years. It's entirely up to whoever looks at your application at the immigration office. There are people who have lived in Japan for 10 or more years who have been turned down for PR, so don't think that it's something you are entitled to after spending 5 years there.

As for giving up your old citizenship when you are granted PR, that's another one of those many laws which Japan turns a blind eye to. They don't actually ask or check to see if you renounce your original citizenship. You can keep it, just don't tell anyone that you did.