I thought Japan doesn't even recognize dual citizenship. After you are 22, you have to choose one country.
http://www.crnjapan.com/issues/en/nationalitylaw_lose_dual_citizenship.html
Oh my goodness! I saw this part:
Secondly, having being domiciled in Japan is a requirement for re-acquiring Japanese nationality. However, there are many cases where a Japanese man marries a woman based in Southeast Asia, then leaves his wife and child returning to Japan alone after their child is born. In this case, since the abandoned wife does not know about the nationality retention system, the child loses Japanese nationality, and, in order to enter Japan, the child will need a visa to enter and stay as a foreigner. In order to obtain such a visa, it is necessary for the child to find his/her father in order to have him as a guarantor. It is quite easy to imagine how difficult it is for the child to discover its father. If the child finds his/her father at all, it is hard to imagine that the Japanese father who has abandoned the wife and child will be willing to become the child's guarantor. This means that it is virtually impossible for the child to be domiciled in Japan. Naturally, in cases of unlawful entry, the child cannot be recognized as having a domicile, as specified in Article 17 of the Nationality Law.
I think that if Japanese men go to Thailand or Cambodia for example, knocks up a Thai or Cambodian woman, then leaves her, Japanese law protects him from having to own up to caring for his mixed child. As you may know it is difficult for a Thai, Russia, or Cambodian to come legally into Japan, and thus, the Japanese man will wash his hands clean, and his wife will not be able to find out as she can't come and tell her.
Glenski, Madame Pappilion, this I think needs to be corrected. Imagine if he doesn't register the child, he could just simply marry another lady in Japan, and under Japanese law, he would be given a clean slate to do so, counting his cambodian or russian wife as irrelevant.
I think I am going to stop dating Japanese men. Just imagine if I got pregnant and then he left, or what if he took the child away, and he was given full custody and I given no rights as a foreigner? I think Japanese men will lose out in dating foreign ladies if every lady finds out about this. Japan can be a country for Japan, and it will be wether they like it or not even for the men when it comes to mate selection.