An apparatus for in situ measurement of the concentration of gas in an aquatic environment and under hydrostatic pressure, comprising: - a device for extracting gas dissolved in a liquid medium, the extraction device comprising: - a tank intended to receive a solvent immiscible in the liquid, - an intake channel for taking in liquid from the liquid medium, - a module (14, 15) for circulating the solvent and the liquid, - and a device (2, 3, 4, 5, 6) for measuring the concentration of gas dissolved in the solvent; the measurement apparatus being characterized in that the extraction device further comprises: - a tubular chamber through which a mixture of the solvent coming from the tank with the liquid coming from the intake channel is intended to pass from one end to the other to transfer the gas from the liquid to the solvent, - a decanter arranged to receive, via an inlet, the mixture of the solvent with the liquid and then to allow the separation by gravity of the mixture into two phases, including a denser first phase, containing the gas-enriched solvent, located at the bottom of the decanter and a lighter second phase, comprising the gas-depleted liquid, located at the surface of the decanter, the first phase then being discharged via a first outlet and the second phase being discharged via a second outlet located above the first outlet, - and a verticality-maintaining element comprising an arm rigidly connected to the decanter and terminated by the weight arranged so that the arm is parallel to the vertical direction of the decanter, in that the measurement device is a device (3, 4, 5, 6) for optical measurement comprising: - a measurement cell (31, 41, 51, 61), - a module (32, 42) that is intended for circulating the gas-enriched solvent and that injects the solvent from the decanter into the measurement cell (31, 41, 51, 61), - a light source (33, 43, 53, 63) that emits an initial light beam and that is oriented so that the initial light beam passes through the measurement cell (31, 41, 51, 61), - a means (34, 44, 54) that is intended for measuring the light intensity and that measures the intensity of the light beam that has passed through the measurement cell (31, 41, 51, 61), and in that it further comprises a sealed compartment (20, 301, 302, 40, 50) maintained at atmospheric pressure and comprising electronic or optical components sensitive to hydrostatic pressure. - a device for extracting gas dissolved in a liquid medium, the extraction device comprising: - a tank intended to receive a solvent immiscible in the liquid, - an intake channel for taking in liquid from the liquid medium, - a module (14, 15) for circulating the solvent and the liquid, - and a device (2, 3, 4, 5, 6) for measuring the concentration of gas dissolved in the solvent; - a tubular chamber through which a mixture of the solvent coming from the tank with the liquid coming from the intake channel is intended to pass from one end to the other to transfer the gas from the liquid to the solvent, - a decanter arranged to receive, via an inlet, the mixture of the solvent with the liquid and then to allow the separation by gravity of the mixture into two phases, including a denser first phase, containing the gas-enriched solvent, located at the bottom of the decanter and a lighter second phase, comprising the gas-depleted liquid, located at the surface of the decanter, the first phase then being discharged via a first outlet and the second phase being discharged via a second outlet located above the first outlet, - and a verticality-maintaining element comprising an arm rigidly connected to the decanter and terminated by the weight arranged so that the arm is parallel to the vertical direction of the decanter, - a measurement cell (31, 41, 51, 61), - a module (32, 42) that is intended for circulating the gas-enriched solvent and that injects the solvent from the decanter into the measurement cell (31, 41, 51, 61), - a light source (33, 43, 53, 63) that emits an initial light beam and that is oriented so that the initial light beam passes through the measurement cell (31, 41, 51, 61), - a means (34, 44, 54) that is intended for measuring the light intensity and that measures the intensity of the light beam that has passed through the measurement cell (31, 41, 51, 61),