A ring-down spectrometry apparatus in absorption saturation condition, for measuring the concentration of a gas through a measurement of the spectrum of a molecular transition of said gas, comprising: • a laser source adapted to emit an electromagnetic radiation beam having a predetermined wavelength in the infrared spectrum, said laser radiation being emitted intermittently or quickly shifted in frequency; • a resonant cavity adapted to contain said gas to be detected and comprising a first and a second reflecting mirror (4a, 4b) so as to form an optical path for the electromagnetic radiation emitted by said laser source, the distance between said first and said second mirror (4a, 4b) being adjustable so as to couple said electromagnetic radiation to said cavity and bring said cavity in resonance or out of resonance with said electromagnetic radiation; • means for varying the wavelength of said radiation emitted by said laser source so as to bring said electromagnetic radiation emitted by said laser in resonance or not in resonance with said molecular transition of said gas; • a photodetector adapted to detect an electromagnetic radiation beam in output from said cavity and adapted to generate a decay signal, said decay signal being a decay signal in laser-gas resonance or laser-gas non-resonance; • an electronic circuit (12, 13) receiving in input the signal in output from the photodetector and adapted to convert it in a signal in input to a processor; the apparatus being characterized by • a processor adapted to receive said decay signal from the photodetector, said processor including: ∘ Interpolation means adapted to perform an interpolation of said decay signal in laser-gas non-resonance so as to obtain one or more calibration coefficients as interpolation parameters, ∘ subtraction means of said calibration coefficients from the decay signal in laser-gas resonance so as to obtain a corrected decay signal in laser-gas resonance; ∘ said interpolation means being adapted to perform an interpolation of said corrected decay signal in laser-gas resonance so as to obtain a concentration of said gas as interpolation parameter; • a diffuser element, disposed between said photodetector and said cavity, adapted to spread said electromagnetic radiation from said cavity on an optical plane of said photodetector. • a laser source adapted to emit an electromagnetic radiation beam having a predetermined wavelength in the infrared spectrum, said laser radiation being emitted intermittently or quickly shifted in frequency; • a resonant cavity adapted to contain said gas to be detected and comprising a first and a second reflecting mirror (4a, 4b) so as to form an optical path for the electromagnetic radiation emitted by said laser source, the distance between said first and said second mirror (4a, 4b) being adjustable so as to couple said electromagnetic radiation to said cavity and bring said cavity in resonance or out of resonance with said electromagnetic radiation; • means for varying the wavelength of said radiation emitted by said laser source so as to bring said electromagnetic radiation emitted by said laser in resonance or not in resonance with said molecular transition of said gas; • a photodetector adapted to detect an electromagnetic radiation beam in output from said cavity and adapted to generate a decay signal, said decay signal being a decay signal in laser-gas resonance or laser-gas non-resonance; • an electronic circuit (12, 13) receiving in input the signal in output from the photodetector and adapted to convert it in a signal in input to a processor; • a processor adapted to receive said decay signal from the photodetector, said processor including: ∘ Interpolation means adapted to perform an interpolation of said decay signal in laser-gas non-resonance so as to obtain one or more calibration coefficients as interpolation parameters, ∘ subtraction means of said calibration coefficients from the decay signal in laser-gas resonance so as to obtain a corrected decay signal in laser-gas resonance; ∘ said interpolation means being adapted to perform an interpolation of said corrected decay signal in laser-gas resonance so as to obtain a concentration of said gas as interpolation parameter; ∘ Interpolation means adapted to perform an interpolation of said decay signal in laser-gas non-resonance so as to obtain one or more calibration coefficients as interpolation parameters, ∘ subtraction means of said calibration coefficients from the decay signal in laser-gas resonance so as to obtain a corrected decay signal in laser-gas resonance; ∘ said interpolation means being adapted to perform an interpolation of said corrected decay signal in laser-gas resonance so as to obtain a concentration of said gas as interpolation parameter; • a diffuser element, disposed between said photodetector and said cavity, adapted to spread said electromagnetic radiation from said cavity on an optical plane of said photodetector.