A fire detector that responds more rapidly than the widely used somke detectors senses increases in the concentration of carbon dioxide associated with a fire. The build-up of carbon dioxide is sensed by measuring the concomittant increase in the absorption of a beam of radiation whose wavelength is located at a strong absorption band of carbon dioxide. The device is considerably simplified by the use of a window (48) to the sample chamber (44) that is highly permeable to carbon dioxide but which keeps out particles of dust, smoke, oil, and water. This permits the fire detector to be built of very inexpensive materials.