[midoridensha]

Technically, it's not a prerequisite, but a requirement. You can't renounce your old citizenship before gaining new citizenship. Japan requires you to renounce within a certain time after gaining Japanese citizenship, with a few exceptions (like Iran where you can't really renounce).

[lmm]

> You can't renounce your old citizenship before gaining new citizenship.

Often you can and in those cases Japan generally requires you to. However it would depend on the citizenship in question.

[midoridensha]

Huh? Renouncing first would make you stateless, which is against UN resolutions. I've never heard anything like this about Japan's naturalization process, just that you're required to renounce after acquiring it (and if you don't and they find out, they'll revoke your citizenship).

[lmm]

Many countries (including mine) allow you to renounce your citizenship but then void that renunciation if you don't acquire a new citizenship within 90 days, so as to prevent the risk of (long-term) statelessness.