A GI insertion section 105 inserts a guard interval into transmission data. Delay addition sections 107-1, 107-2 set a delay time in the transmission data. An arrival time calculation section 115 calculates for each directivity an arrival time after data is transmitted from the other party of communication until the data is received from received data of each directivity. A delay time determining section 116 calculates a difference in the arrival time between transmission data transmitted with two directivities and sets the calculated arrival time difference in the transmission data to be transmitted with the directivity corresponding to the smaller arrival time as a delay time. A GI length determining section 117 sets a minimum arrival time of the calculated arrival times as a guard interval. This allows a transmission rate to be improved by shortening the length of a guard interval in a radio communication system to which a system of transmitting an OFDM signal with a directivity is applied.