In this paper, we propose a synchronization scheme to obtain frequency diversity in a WLAN system which simultaneously uses the multiple bands such as 5 GHz, 2.4 GHz and 920 MHz simultaneously. The proposed scheme is to combine the output of correlators after compensation for the difference of the center frequency on each frequency band. The proposed scheme obtains a large frequency diversity gain by joint synchronization processing across multiple bands, and improves the accuracy of synchronization. Simulation results show that the proposed scheme can achieve a better performance of synchronization than a conventional scheme which performs synchronization process only in a band.