seems to be very difficult.
It looks like you should add a romanji and a kanji/hiragana/katakana version.
Usually the japanes address is in one line, the international has some more.

My sources: http://www.upu.int/fileadmin/documentsFiles/activities/addressingUnit/jpnEn.pdf
http://www.upu.int/uploads/tx_sbdownloader/descriptionPostcodesAddressingAddressElementsEn.pdf

http://de.wikipedia.org/wiki/Postadressen_in_Japan
http://en.wikipedia.org/wiki/Japanese_addressing_system

Has someone any experience with sending letters to Japan?
What is the best way to avoid problems?

@IceT

Dont worry, it is not difficult, I have over 30 friends in Japan and we often exchange small gifts.
I just write the romanji version and my letters always get there OK.

Here is a real example :

1-29-9 Kitarokkodai
Nishinomiya-shi
651-1513
Hyogo
Kobe
Japan

It's not difficult. You write it using romaji, that's it.

Roman lettering is an international standard for international mail.

I send mail to Japan all the time for my business.

Here's the USPS page on the subject: https://www.usps.com/send/international-addressing-tips.htm

Note that they say "English" but they really mean Roman lettering. (I think they just say "English" because Americans would be confused if you said "Roman lettering").

While this is the USPS and I know you're in Germany, obviously mail going *into* Japan all has to be addressed the same way, and this is how. You always use Roman lettering when addressing anywhere - otherwise your own post office would not know if the address was even valid.

Well, it does no harm if you write both versions, if you are familiar with writing in japanese.
But they also get along well if you only write the romaji-version. I've sent parcels in both versions (romaji only and romaji+hiragana/katakana/kanji) and they all arrived