Nigeria does have a lot of oil, and I believe that Japan suffers them to a degree to maintain friendly relations with their embassy in Tokyo and to keep the oil flowing. Most of them come over as tourists or to study Japanese. I asked one one night after he tried to hawk me into his rip off bar how he is allowed to stay. The answer has been pretty standard amongst all the ones that I've made an effort to speak to.    The standard formula for staying in Japan if you're a Nigerian or a 3rd-world African lout: Find a Japanese girl dumb enough to have intercourse with you   Make sure that said girl is also stupid enough to have sex with you without the protection of a condom   Impregnate said Japanese girl   Make an offer of marriage so that the child is not a bastard (I use that term loosely in this instance)   Apply for and receive a dependent spouse visa    Viola! Now you are free to go around and pick up other dumb Japanese girls that for whatever reason find African muds cool and/or attractive while the wife sits at home. Pretty easy to do when you are working the graveyard shift as a hawker for dodgy strip or hostess clubs in Roppongi.