In this paper, we describe a novel method for generating a highly repetitive optical pulse train using mode-locked pulses and the group-velocity dispersion (GVD) of optical fiber. An optical pulse train at a multiplied repetition frequency of an initial mode-locked pulse train is obtained by adjusting fiber length in accordance with the mode-locking frequency and the fiber's GVD. A subterahertz optical pulse train (98-196 GHz) was successfully generated with low pulse intensity fluctuation.