Gas cells are widely used in THz and IR absorption spectroscopy. However, reflections of an optical wave on cell input and output windows cause a gas sample measured spectrum distortion due to the Fabry-Perot effect. The hardware approach to decrease the Fabry-Perot effect in the THz spectral range is suggested. The approach is based on using a standard gas cell and the additional movable optically transparent window. To illustrate usefulness of the suggested approach, a simulation of registration of the sulfur dioxide absorption band near 660 GHz in the gas mixture consisted of 0.1 % SO2 and 99.9 % N2 was conducted. The absorption band profile fluctuations due to the Fabry-Perot effect were shown to be reduced by almost 4 times with the suggested approach implementation in relation to the default gas cell. This approach can be combined with other hardware and software variants of the Fabry-Perot effect suppression.