The device comprises a laser source which emits a beam of a predetermined wavelength towards an optically transparent closed container and detectors arranged to detect the laser beam which is attenuated by the gas absorption. The detectors provide the first data of absorption representative of a first absorption spectrum of the gas including distorted absorption lines and noise. The invention implements a method of calculation aimed at receiving and elaborating said first data. The output of the operation are parameters that represent a second absorption spectrum free of noise and distortion in the absorption lines. From these parameters we are able to determine the gas pressure and concentration in the container.