In this paper we present a description of the laser gas analyzer operating within the spectral range from 2.7 to 4.5 mkm with the resolution of 0.2 cm-1. The gas analyzer is comprised of IR laser generator, transceiving system, control system, and horizontal path 0.1–1 km long equipped with mirror reflector. The continuous wavelengths tuning has been performed by mixing second harmonic of Nd3+:YAG laser radiation with the radiation from a sinchronously pumped dye laser in a nonlinear optical crystal (KDP). The concentration sensitivity of the gas analyzer is at the level of 0.1 ppm (for methane on a 100 m long path). By way of example the path measurements of absorption spectra of methane and water vapor demonstrate the possibility of analyzing the minor atmospheric constituents at the background level of concentration with the use of the instrumentation proposed.