I've been reading all around about that topic and aparently this is what I understand you have to do if you enter with your car in Japan and your country IS part of the 1949 convention:

1) Send a copy of your CDP (carnet de passage) by fax to the JAF (Japanese automotive federation) to one of those fax numbers (for instance if you enter by Sakaiminato to the Hiroshima one):

JAF For Visitors Bringing Vehicles with Carnet de Passages en Douane
Sapporo: +81-11-857-7177 Miyagi: +81-22-783-2821
Tokyo: +81-3-5440-2564 Aichi: +81-52-872-1164
Osaka: +81-72-645-1666 Hiroshima: +81-82-272-0324
Kagawa: +81-87-869-9316 Fukuoka: +81-92-841-7815

1.1) If you need more drivers to be able to drive the vehicle, ask on this very same fax for the corresponding forms that you will later have to present in customs.

2) Once you have arrived, go the local branch of the JAF and present your CDP for validation (will be aparently fast if you sent your docs beforehand, if not, they'll do it but will take more time):
JAF Regional Headquarters & Offices : CHUGOKU
JAF Shimane Branch Office 690-0011 1092-1 Higashitsuda-cho Matsue-shi Shimane 0852-25-1123 (example for Sakaiminato)

3) Obtain insurance

4) Clear customs



Question about 3) :

Where can I get a car insurance for driving my car in Japan beforehand ? Is it possible or should I just have to buy it when I get there ?
Travellers – Apexmoto Inc
Can a company like this one get it for me ?


Thanks everyone !

Usually, insurance is available at any car or motorcycle dealer.
If that company says they can do it for you in advance, ask them.
It may save you the hassle of doing it in Japan.

__________________
Japan touring information

Ok, I see, I was in doubt of whether car insurance could be bought in bike stores also, thanks for that !

Apexmoto can issue Jibaiseki and Ninihoken, in advance. For German and Swiss registered vehicles, we will only do this one, as technically, they are illegal on Japanese roads, with the exception of trips to get the vehicle registered.

You can email sales@apexmoto.jp

We are licensed insurance agents, currently offering insurance through Mitsui and Zutto. Our license covers all motorised vehicles, but yes we are a bike shop