© 2018 Universiti Teknikal Malaysia Melaka. All rights reserved. A compact and low-cost gas sensor using absorption spectroscopy for Carbon Dioxide (CO2) measurement is presented. The sensing principle is based on open-path direct absorption spectroscopy in the mid-infrared range. The improved reflective structure of optical gas sensor consists of low cost and compact components, such as filament emitter, multispectral pyroelectric detector, Calcium Fluoride (CaF2) window and aluminium curve reflective surface. In the previous investigation, the optimized gas cell structure was simulated using ZEMAX®12 software prior to fabrication for measuring CO2 gas concentration. The developed gas sensing system using the optimized gas cell structure has shown the capability of accurately detecting CO2 concentration. The sensor utilizes a CaF2 narrow bandpass (NBP) filter for detection of CO2 gas with no cross-sensitivity with other gases present in the gas cell. The repeatability of sensor¿s response of detecting CO2 was tested with response times were calculated as being less than 1 second.