Synthetic bandwidth radar has been proposed for high-range-resolution. The transmitted-frequency and down-conversion frequency of this radar are shifted by a fixed step frequency from pulse to pulse. In order to improve a range resolution, received signal are synthesized by taking IFFT. When the step frequency size is higher than a bandwidth of a pulse, range measurement result is ambiguous. In this paper, we propose a high-range-resolution method by using Wiener filter for synthetic bandwidth radars. Our proposed method gives an unambiguous range measurement even though the step frequency size is higher than the bandwidth of the pulse. For numerical simulation results, range resolution was improved without range ambiguity. (4 pages)