The criteria for naturalization are provided in Article 5 of the Nationality Act:
1. Continuous residence in Japan for five years or more
2. At least 20 years old and otherwise legally competent
3. History of good behavior generally, and no past history of seditious behavior
4. Sufficient capital or skills, either personally or within family, to support oneself
5. Stateless or willing to renounce foreign citizenship
Source:
Japanese nationality law - Wikipedia, the free encyclopedia