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 by contacting the local Japanese consulate or embassy in your country or visiting the website of Ministry of Foreign Affairs of Japan at:
http://www.mofa.go.jp/j_info/visit/visa/
Note: Please note GRET is not authorized to assist with the VISA process beyond providing the Notification of Acceptance Letter and Invitation Letter (after registration) issued by the GRET Committee Board. Should your application be denied, GRET Committee cannot change the decision of the Japanese Ministry of Foreign Affairs, nor will GRET Committee engage in discussion or correspondence with the MOFA or the Embassy of Japan on behalf of the applicant.