Please look at the list of the countries that have the visa exemption agreement with Japan. If your country is included in the list, you usually do not need to apply for an entry visa to Japan.
If your country is not included in the list, you need to obtain a short-term stay visa (“tanki-shouyou visa” in Japanese) from the embassy of Japan in your country.
Read carefully the Japanese visa guide provided by the ministry of foreign affairs. For entry into Japan, a VISA may be required for attendees from some countries. Please make sure whether you need to apply for a VISA or not on the web site of Ministry of Foreign Affairs of Japan at : http://www.mofa.go.jp/j_info/visit/visa/index.html.