A photonic crystal based on a macroporous silicon structure has been fabricated and successfully used for the detection of carbon dioxide. In this paper, the device and the measurement results are presented. The sensing device here described uses an optical approach to the detection of the gas. The use of a photonic crystal allows creating a compact device working in the medium infrared spectral range. A 700 nm square lattice macroporous structure was fabricated by electrochemical etching, creating a photonic gap centered at the 4.2 µm, a CO2 absorption line. The obtained results rely only on the absorption spectra measurement.