A method to monitor atmospheric gas concentration employing a tunable diode laser is proposed. An apparatus was built and experiments were made. The tunable diode laser employed was of'PbSnTe with its wavenumber about 1200 cm(-1) at a temperature of 77 K. Methane concentration in laboratory atmosphere was measured on the experiments. The inquired gas-density is obtained by a hardwired microcomputer incorporating the measured absorption spectrum over a prescribed range instead of an absorption at a single wavenumber. By this, drift type noises are suppressed. After an analysis based on experimental data, an attainable sensitivity for methane was found to be 1 ppb.m. This value may increase if an etalon fringe interference exists. The natural concentration, about 1 ppm, of atmospheric methane was actually measured with a short optical path of only one meter.