About the conditions to be naturalized Japanese citizen, Naturalization
It differs depending on your residence activities in Japan.
About Japanese abilties
In principle,the abilities of Japanese language of about second grader of the elementary school is a prerequisite for the Naturalization permission, if your residence status is work, spouse, long term or dependent visa holder.
There are a few examinations about your ability of Japanese language in the examination. (Excepted special permanent holder and child , and some case.)
Let’s check the provisions of the Nationality Law.
Mainly,foreigners who are resident status as working visa, long term or Dependent, etc,
Article 5 Japanese Nationality Law
(1) The Minister of Justice may not permit naturalization for a foreign national who has not met the following conditions:
(i) Having continuously had a domicile in Japan for five years or more;
（It means that it will be required strongly that you have been living in Japan with the thing that you have an address in Japan continuously, and you did not travel to abroad a long period of time. Specifically, it was whether more than 180 days a year or not. In case of it was 180 days a year without unavoidable reason. , it will be a bit problem.)
(ii) Being twenty years of age or more and having the capacity to act according to his/her national law;
(In case of that you will apply it for the whole family in same time; this is not required for your children. And there are other cases. In case of spouse of Japanese, Article 7 of the Nationality Law is applied, so this requirement relaxed.)
(iii) Being a person of good conduct;
(It is must that you have no criminal record. If you have anything, it required already past 5years or 10 years, upon the case of criminal basically.)
(iv) Being able to make a living through his/her own assets or abilities, or through those of a spouse or of another relative his/her making a living;
(v) Not having a nationality or having to give up his/her nationality due to the acquisition of Japanese nationality;
(Before your application will be permitted in the process of the examination, you will be required that go to your embassy to do the proceedings of renounced nationality.)
(vi) On or after the date of promulgation of the Constitution of Japan, not having planned or advocated the destruction of the Constitution of Japan or the government established there under with force, and not having formed or joined a political party or other organization planning or advocating the same.
(2) In cases where despite the foreign national's intention, he/she is unable to give up his/her nationality, the Minister of Justice may permit naturalization if special circumstances are found concerning a familial relationship or circumstances with a Japanese citizen even if that foreign national has not met the conditions listed in the preceding paragraph, item (v).
Spouse of Japanese
Article 7 Japanese Nationality Law
The Minister of Justice may permit naturalization of a foreign national with a spouse who is a Japanese citizen, said foreign national continuously having a domicile or residence in Japan for three years or more and who currently has a residence in Japan even if that person does not meet the conditions of Article 5, paragraph (1), item (i) and item (ii). The same shall apply to a foreign national with a spouse who is a Japanese citizen, for whom three years have elapsed since the date of their marriage, which foreign national has continuously maintained a domicile in Japan for one year or more.
For example if you have been living in Japan 3 years as a student, and then married to Japanese, you will be eligible to apply for this naturalization immediately.
Otherwise you have been living in Japan more than 1 year, after you have married in abroad and living in there more than 3 years, and then you can immediately.
If you have a fiance who is living with you now, and then she/he has a spouse (even if the marriage situation is broken ) it will be weak point to examination for Naturalization application. There is a possibiity the applicaiton is not accepted.