In cognitive radio systems, spectrum sensing schemes for detecting primary users in a licensed spectrum are essential to utilize the spectrum effectively. As a matter of fact, sensing accuracy is regarded as the most important factor for spectrum sensing with the intension of sufficient protection for the primary user. In this paper, we propose a new spectrum sensing scheme which exploits the cyclic prefix of an orthogonal frequency-division multiplexing (OFDM) symbol as well as idle period containing no information symbols. Redundant information in the cyclic prefix enables spectrum sensing to be more accurate and efficient. With analysis and simulation, we show that the proposed scheme can improve sensing accuracy and increase system throughput compared with the conventional system which uses only idle period.