Originally Posted by joejones

This depends on your nationality. Americans are given a grace period after naturalizing in Japan, because the US government will not allow them to renounce citizenship if it would make them stateless.

In other countries, such as Britain, you CAN expatriate yourself while a naturalization application is pending somewhere else, so the Japanese government requires proof of expatriation as a prereq to naturalization (see here for an example).

That said, even Americans are supposed to expatriate themselves within two years after naturalizing in Japan. What happens if you don't is a separate question.