Showing Japanese passport in Japan?

Hi, I am living in US and have a Greencard. I am a Japanese Citizen.

I have questions about Dual citizenship US and Japanese. I read many comments about using both passport(US: Come and go = use US one and Japan: Come and go = use Japanese one).
I go back Japan twice a year. Every time I leave from Japan to US, I have to show my Greencard. (Narita)
Not only that, I need to show it the custom section as well. (Not the immigration, after pick up the luggage)
So I don't understand how they can leave Japan using Japanese passport to come back to US. After I get the US citizen, of course I don't have Greencard any more.

I am trying to get the US citizen ship but I don't want to loose Japanese one. I wasn't clear the passport issue when I go to Japan. I hope my questions is clear...
Does someone help me on this?
Thank you very much.

Like you my mother is a citizen of Japan living in Palo Alto and each time she visit Japan she is asked to show her Green Card.

There is no advantage for you to get an U.S. citizenship just to visit Japan - except with a U.S. passport you won't be asked to see the Green Card. That is all.......!

Dual Citizenship:

Because your birth country is Japan, you are a citizen of Japan.
Your children birth country is U.S. , so they are an American citizen, just like me.

However the Japan citizenship laws allow your children or I to apply for a Japan citizenship because you/husband or my mother
father are a Japan citizen,.... But Japan laws also mandate that any person (living in Japan) with a Japan citizenship and with citizenship of another country he/she must declare to keep one citizenship and give up the second citizenship before the age of 22 years old.

One reason people with a dual citizenship use Japan or U.S. passport to leave - and use U.S. or Japan passport to enter - they don't want to give-up one of their citizenship. Because if American citizen 'renounced' his citizenship, then, U.S. Government will treat he/she just like any foreigner is treated if he changed his mind to go back to live in U.S. - U.S. Immigration will treat him/her as a new immigrant to U.S. and make them start the immigrating procedures just like you and my mother had to do.

If you don't want to lose your Japanese citizenship, don't get American citizenship. It's that simple.

Like it was stated previously, being born with dual citizenship seems to be a whole different question. However, since you were born with Japanese citizenship, you automatically renounce your Japanese citizenship as soon as you acquire American citizenship.

As for the specific issue regarding your passport, I (I was born a dual citizen, still hold both Japanese and American passports at Age 25) show my Japanese passport upon entering and leaving Japan, American upon leaving and entering the US. In the five times I've done this over the last couple years, I've never been asked by Japanese officials why I don't have a stamp in my Japanese passport or anything like that. Have I just been lucky? I'm not sure, I just tell them every time that I live in American and am just visiting relatives in Japanese for a short period.

In your particular case though, depending on how much you want to keep Japanese citizenship, I would strongly advise against getting American citizenship.

I don't see what the problem there is?

It is quite simple, people show their Japanese Passport leaving Japan and entering Japan, they show their US Passports entering the USA and leaving the USA. It is a system that has worked for decades, and people continue to do it.

Don't get the US citizenship untill you are ready to give up the Japanese one. I am in the same situation like yours, but never had any trouble like that.... The only time I show my greebcard is when I enter US.

There are many older threads about this issue. Do a search and they should turn up. Try searching for dual citizen or passport. If you have trouble, please post here.

You might want to consider traveling on return tickets, that would remove the need to show the green card.