That converting works is called "Gai-Men Kirikae(OΖΨΦ)" in Japanese, read foreign langs guide(PDF).
Gai is short of Gaikoku(O) = foregin contries,
Men is short of Unten-Mekyo(^]Ζ) = driver's license,
Kirikae is coming fr Kiri-kaeru(ΨθΦ¦ι as Verb) = switching/coversion.
This page is written by Metropolitan Police Department(called "Keishi-cho/x‘" in Japanese)
only for people who living in Tokyo as one prefecture of 47, http://www.keishicho.metro.tokyo.jp/menkyo/menkyo/kokugai/kokugai05.ht...
but other all 46 prefectures is mostly same.https://www.npa.go.jp/link/prefectural.html
This is Kanagawa Prefectural Police Department(Kanagawa-ken keisatsu/_ήμ§x@)'s one.https://www.police.pref.kanagawa.jp/mes/mes83046.htmhttps://www.police.pref.kanagawa.jp/eng/e_mes/eng83000.htm#exchanging
This is for Kyoto.http://www.pref.kyoto.jp/fukei/menkyo/s_men1/gaimen/index.htmlhttp://www.pref.kyoto.jp/fukei/foreignlanguage/index.html
This is for Okinawa without foreign lang page/guide. http://www.police.pref.okinawa.jp/docs/2015030100253/
This is JAF's one. (JAF : Japan Automobile Federation)http://www.jaf.or.jp/inter/translation/http://www.jaf.or.jp/e/translation/switch.htm
Anyway, all the procedures of Japanese driver's license relations are managed
by the driver's license department(unten-menkyo-ka/^]ΖΫ)
of the ministry of transportation(koutsuu-bu/πΚ) ofeach prefectural police depertment(keisatsu/x@).
So, call a "licensing center(unten-menkyo center, or shikenjo/^]ΖZ^[ or ±κ)"
of each prefectural police department, or if close, go directly and consult at counter.
If already registered for resident, you should go consult at(or call to) city/ward/town/village hall ASAP.
The staff of the city/ward/town/village hall doesn't know the rules of driver's license well,
but if you tell the important words of "OΖΨΦ" and more(related URLs all), will be able to investigate further.
From the driver's license of Nigeria, it seems that exams of "knowledge(paper exam)"
and "driving skill(driving exam)" but it seems that it will NOT be exempted, but they will also investigate this.I lives in Hokkaido prefecture very far from the embassy.
Oops, I was in the process of writing an answer.
There are EN page in Hokkaido Police depertment web, but about "OΖΨΦ" is written in only Japanese.https://www.police.pref.hokkaido.lg.jp/english/english-top.htmlhttps://www.police.pref.hokkaido.lg.jp/guide/menkyo/gai_menkyo-kirikae...
Getting yr driver's license is NOT a Goal, registering and possessing own car is a different story
(proof of garage, automobile tax, car insurance, automobile inspection, and more),
so, you have to study more knowledge of next stages if you could, pls don't forget it.