An apparatus and method for transmitting data on a plurality of frequency sub-channels for a plurality of successive symbol periods through a plurality of transmit antennas in an OFDM system. In the data transmitting apparatus, an S/P converter converts information symbols received from a data source to an information symbol vector, a coder generates at least one code symbol vector using the information symbol vector in at least one symbol period, an IFFT unit generates at least as many transmission signal vectors as twice the number of code symbol vectors generated in the coder for the one symbol period, a plurality of P/S converters each convert the transmission signal vectors to a transmission signal stream, and a guard interval inserter inserts a guard interval into each of the transmission signal streams received from the P/S converters.