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 (anki-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.
Important information regarding invitation letters (please read):
Please note CPEEE Conference and the organizing committee of the conference are not authorized to assist with the VISA process beyond providing the Notification of Invitation Letter issued by CPEEE Conference and conference Committee Board. Should your application be denied, CPEEE Conference and the organizing committee of the conference cannot change the decision of the Ministry of Foreign Affairs, nor will CPEEE Conference and the organizing committee of the conference engage in discussion or correspondence with the MOFA or the Embassy on behalf of the applicant. The registration fee cannot be refunded when the VISA application of individual being denied.
It takes time for the Embassy to process the VISA application, please register as early as you can to make sure you have enough time.