Carbon dioxide (CO₂) is an atmospheric trace gas; therefore its accurate sensing is of great interest. Optical sensors exploiting the midinfrared (mid-IR) light absorption of CO₂ provide high sensitivity and are widely used in medical diagnostics, atmospheric monitoring, remote sensing, and industrial applications. In this work, we demonstrated accurate CO₂ gas sensing at a 4.2-μm wavelength. In addition, detecting the weak mid-IR molecular absorption bands of gases at low concentrations requires using increased optical path lengths. The most obvious method that can expand the potential beam path in a spectroscopic system is to use a longer linear gas cell, which in some situations may be adequate; however, space and volume requirements need to be considered. A circular multireflection (CMR) cell was used to reflect the radiation back and forth through the sample medium multiple times, thus greatly reducing the size footprint compared with a linear cell of equivalent optical path length. A CMR cell was designed and constructed to allow for multireflections within the cell. The optical alignment of the cell and the convenience of changing the optical path length by adjusting its position with respect to the incident light beam were also used to maximize the advantages of the device. The cell has an inner diameter of 6.5 cm, and the path length can reach up to 123.5 cm, which is equivalent to 19 beam passes of reflections. This work will be used as the groundwork for designing an instrument for the high-resolution measurement of CO₂ gas in planetary atmospheres.