A device for measuring the concentration of a gas dissolved in an electrical insulation oil comprises a measuring member (2), internally defining a measuring chamber (3) and a conduit (4) to access the measuring chamber (3), a membrane (6) permeable to gases, inserted inside the conduit (4) for insulating the measuring chamber (3) against the oil and allowing gas to pass from the oil to the measuring chamber (3), a sensor (20) located in the measuring chamber (3) for measuring the concentration of gas in the measuring chamber (3), and a reinforcing grid (8) positioned in the conduit (4) for preventing deformation of the membrane (6), where the grid (8) is located at a position adjacent to the membrane (6) on the side opposite to the measuring chamber (3), and has at least one through section (9,10) for allowing the oil to exert pressure directly on the membrane (6).