An OFDM transmitter which transmits an OFDM signal by a frame having a plurality of transmission symbol periods each constituted of an effective symbol period for transmitting an information bit and a variable guard interval, wherein the frame length is not changed, the OFDM transmitter comprising: a guard interval length decider configured to decide a guard interval length in accordance with a feedback signal from an OFDM receiver; characterized by a guard interval length changer configured to change the guard interval length in each transmission symbol period, in accordance with the decided guard interval length; wherein the guard interval length changer comprises • an in-frame transmission symbol number calculation section configured to calculate an in-frame transmission symbol number based on the frame length, the decided guard interval length, and the effective symbol period; • a adjustment time calculator configured to calculate an adjustment time necessary for maintaining the frame length constant, in accordance with a current guard interval length, the effective symbol period, and the in-frame transmission symbol number; • a selector configured to select one of the decided guard interval length and the decided guard interval length having the calculated adjustment time added thereto in each transmission symbol period, the decided guard interval length having the calculated adjustment time added thereto being selected for a plurality of transmission symbol periods; and • a guard interval adder configured to add the selected guard interval length in each transmission symbol period, thereby changing a guard interval length without changing the frame length. a guard interval length decider configured to decide a guard interval length in accordance with a feedback signal from an OFDM receiver; characterized by a guard interval length changer configured to change the guard interval length in each transmission symbol period, in accordance with the decided guard interval length; wherein the guard interval length changer comprises • an in-frame transmission symbol number calculation section configured to calculate an in-frame transmission symbol number based on the frame length, the decided guard interval length, and the effective symbol period; • a adjustment time calculator configured to calculate an adjustment time necessary for maintaining the frame length constant, in accordance with a current guard interval length, the effective symbol period, and the in-frame transmission symbol number; • a selector configured to select one of the decided guard interval length and the decided guard interval length having the calculated adjustment time added thereto in each transmission symbol period, the decided guard interval length having the calculated adjustment time added thereto being selected for a plurality of transmission symbol periods; and • a guard interval adder configured to add the selected guard interval length in each transmission symbol period, thereby changing a guard interval length without changing the frame length. • an in-frame transmission symbol number calculation section configured to calculate an in-frame transmission symbol number based on the frame length, the decided guard interval length, and the effective symbol period; • a adjustment time calculator configured to calculate an adjustment time necessary for maintaining the frame length constant, in accordance with a current guard interval length, the effective symbol period, and the in-frame transmission symbol number; • a selector configured to select one of the decided guard interval length and the decided guard interval length having the calculated adjustment time added thereto in each transmission symbol period, the decided guard interval length having the calculated adjustment time added thereto being selected for a plurality of transmission symbol periods; and • a guard interval adder configured to add the selected guard interval length in each transmission symbol period, thereby changing a guard interval length without changing the frame length.