half-japanese, dual nationality

Hello! I'm 23 (half-japanese) and have dual nationality. I will probably have to leave to japan in a few days. I first have to buy my ticket but will have to choose the name witch I will use for travelling, as my name on the japanese passport isn't the same as on my belgian one. Japanese passport: mother's name (she is japanese). My passport is expiring in less than a month. Will I have problems for entering the country?
I'd like to renew my passport in Japan, is it possible? And if not, how would I have to come back home? Thank you very much!!

I had a passport that was expiring and had to travel on short notice and I asked the Japanese embassy: A passport need to be valid on entry of Japan, Japan has no problem if it expires during your stay. However, usually an expired passport creates problem upon retuning home and therefore it is of course recommended to have a valid passport throughout your stay.

You usually renew your passport in the country of residence. In your case, the Japanese embassy in Belgium is the place to renew your passport. My experience with renewing my wife's passport with the Japanese embassy, the process is rather fast so it maybe worth taking a trip to Brussels and get it renewed prior to your trip. On the other hand, you could just use your Belgium passport and be fingerprinted at the airport and have a 90 day limit to your stay.

Thank you very much Kappa for your answer!
I'd prefer not been fingerprinted...anyway I guess there is no choice :)... Actually, I went to the japanese embassy in belgium since at that time, I thought I still could renew my passport, but then they asked me if I had a foreign nationality (I told them the truth). After this, they said something like I've had to choose my nationality before my 22nd birthday (I'm 23 now)...therefore I'm wondering if I still have a choice right now...and what would happen if I decided to stay a japanese national...? Maybe all this would be easier to do in japan...
It's quite risky leaving with my japanese passport as I'm returning back home a week before the expirery date...and if I can't renew my passport, they probably will not let me leave... dunno :)