A low-cost NO₂ trace concentration measuring device based on a cavity ring-down spectroscopy (CRDS) technique is presented. We theoretically and experimentally demonstrate that, if the nonresonant direct reflection is effectively isolated, an inexpensive broadband diode laser (LD) can excite multiple cavity modes and stabilize the cavity output signal with an optical feedback (OF) method. To improve the measuring accuracy for low concentrations of NO₂ gases, we also design and machine a cavity with several mixing rooms. Several standard concentrations NO₂ gases are used to test the apparatus. The results prove a great agreement between measured and theoretical values over the full range.