As an effort to overcome the physical limitation of scaling down the size of transistors and to implement ultra-low-power consumption in integrated circuits, the use of negative quantum capacitance has attracted significant attention, because it can simply enhance the gate capacitance in field-effect transistors. Among various two-dimensional materials, topological insulators (TI) have immense potential to achieve the effect of negative quantum capacitance, because they can be included in the gate stack of conventional metal oxide semiconductor (MOS) field-effect transistors. In order to verify their advantages for various gate capacitance values, we have fabricated a MOS capacitor with a TI. At room temperature, the capacitance enhancement (i.e., total gate capacitance higher than the theoretically estimated geometric capacitance) of the capacitor was experimentally observed near the depletion region. The effect of negative quantum capacitance, which is caused by the electron system on the surface of TI, demonstrates that the gate capacitance can increase in the range of frequency, i.e., 50-100 kHz.