The errors in determination of the methane concentration in ambient air, connected with the difference in temperature and pressure inside the measurement cells, addition of water vapor, carbon dioxide, and ethylene in the gas probe, and the random noise, are estimated numerically. It is shown that the presence of the foreign gas spectral line in the spectrum under recording together with the analytical methane line can result in overestimated or underestimated values of the methane concentration relative to the true one. The total error in determination of the methane concentration under atmospheric conditions, caused by the presence of water vapor or carbon dioxide in the air probe, as well as by the random noise is less than 1%. Inside the measurement cells, variations of the temperature and pressure, limited by 1° and 1 Torr, respectively, yield an error not higher than 1.5%. At equal concentrations of methane and ethylene in the air probe, the error of determination of the methane concentration can reach 15%.