The advantage of frequency-domain terahertz (THz) spectroscopy using frequency-tunable photomixers is a high spectral resolution, as high as 1 MHz when combined with fiber stretchers as previously reported. In this technique, however, data acquisition was sequentially made at each frequency, thus taking long acquisition time. To improve this drawback, a novel feedback-controlled rapid-scan technique was developed in this study. With this technique, we attained an acquisition rate of 18 points/s without degrading spectral resolution, which is more than two orders of magnitude faster than the previous rate.