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In an orthogonal frequency division multiplexing (OFDM) system, a guard interval (GI) is used to remove the inter-symbol interference (ISI) due to a multipath channel. When the length of the GI is shorter than the maximum delay of the multipath channel, the ISI occurs. On the other hand, a long GI decreases transmission efficiency. In this paper, we propose an adaptive control method of the guard interval length. A channel impulse response is obtained by applying inverse fast Fourier transform (IFFT) to the estimated frequency-domain channel response. The maximum delay is then estimated from the channel impulse response, and the optimal length of the GI is decided from the maximum delay.