You'll need to check the
list of the countries
that have the visa exemption agreement with Japan. If your country
is included in the list, then you will not be required for a visa
when entering Japan under normal circumstances. If your country is
not included in the list, you'll need to obtain a short-term stay
visa from the embassy of Japan in your country.
Please read the
Japanese visa guide provided by the ministry of
foreign affairs carefully. And be informed that it is the embassy of
Japan in your country that makes a decision on issuing a visa.
Whether the attendees need Letter of Invitation fully depends on the
relations between countries. The conference organizers hereby
reiterate that we are neither able to nor authorized to decide the
relations between countries.