Japan has visa exemption agreements with 67 countries. First check whether you are citizen of one these countries.
If this is not the case, you need to apply for a short-term visa for participation to an international conference. Some general information is available from the Ministry of Foreign Affairs, but you should contact your consulate for details.
In case this is needed, we can send you some supporting documents required for the application. Please send a request to the local organization committee, after you register for participation and pay your participation fee. Note that the whole procedure can be lengthy, so be sure to send your request more than 1 month before your trip to Japan.
For all information contact: icfp-visa at researchr.org