Double-beam long-path gas analyzer "Trassa" consists of the LUS type CO2-laser with length-controlled two wave spectrum, the Cassegranian telescope, the cat's-eye retroreflector, data processing and control units. The last is based on the AFC principle and combines functions of switching and power equalization of work and reference lines. The optics arranging and parameters have been chosen to minimize calibration errors. The gas analyzer is intended to use for a chemical plant environmental control service.