A communication system using an OFDM includes a data creation section for coding data to be transmitted and mapping the data, a null symbol insertion section for filling a null symbol into a no-data subchannel if the number of subchannels containing the mapped data is small for the band assignment, and a symbol interleave section for performing symbol interleave in the whole user assignment band and inserting a known training symbol and pilot symbol into the determined symbol position of the user assignment band are included and symbols are placed such that signal phase change is continuous in the same subcarrier between symbols and carrier sense is executed at the positions of the symbols where the signal phase change is continuous.