A capacitive level sensor device, for detecting the level of a medium contained in a container, in particular a tank, wherein the device comprises a circuit support having a generally flattened shape, which extends longitudinally substantially according to level-detection axis, the circuit support having a first longitudinal end and a second longitudinal end, the circuit support having a supporting structure (201, 202, 203) with a first major side and a second major side opposite to each other, as well as two opposite longitudinal edges, wherein the circuit support has, in a detection region thereof that includes said first longitudinal end, at least one first plurality of first capacitive elements, which comprise at least one first array of first electrodes, preferably spaced from one another along the level-detection axis, the first electrodes being made of an electrically conductive material and arranged at least in part in a position corresponding to the first major side of the supporting structure (20 1, 20 2, 20 3) of the circuit support, wherein the device has a casing body which comprises an electrically insulating and fluid-tight detection portion, which covers the detection region of the circuit support, wherein the detection portion of the casing body comprises an overmoulded outer coating, made of a first electrically insulating polymeric material, preferably selected from among thermoplastic polymers and thermosetting polymers, the first electrically insulating polymeric material defining an outer surface of the casing body which is designed to be in contact with the medium the level of which has to be detected, characterized in that - between at least part of the overmoulded outer coating and the first electrodes there is set at least one intermediate insulation layer (20 3, 40, 40', 50; 20 3, 50; 20 3, 40, 40'; 40, 40', 50; 20 3; 40, 40'; 50) made of an electrically insulating material different from the first electrically insulating polymeric material, - the at least one intermediate insulation layer comprises a layer made of a material selected from among materials that include silicon, or derivatives or compounds thereof, and materials that include fluorine derivatives or compounds. wherein the device comprises a circuit support having a generally flattened shape, which extends longitudinally substantially according to level-detection axis, the circuit support having a first longitudinal end and a second longitudinal end, the circuit support having a supporting structure (201, 202, 203) with a first major side and a second major side opposite to each other, as well as two opposite longitudinal edges, wherein the circuit support has, in a detection region thereof that includes said first longitudinal end, at least one first plurality of first capacitive elements, which comprise at least one first array of first electrodes, preferably spaced from one another along the level-detection axis, the first electrodes being made of an electrically conductive material and arranged at least in part in a position corresponding to the first major side of the supporting structure (20 1, 20 2, 20 3) of the circuit support, wherein the device has a casing body which comprises an electrically insulating and fluid-tight detection portion, which covers the detection region of the circuit support, wherein the detection portion of the casing body comprises an overmoulded outer coating, made of a first electrically insulating polymeric material, preferably selected from among thermoplastic polymers and thermosetting polymers, the first electrically insulating polymeric material defining an outer surface of the casing body which is designed to be in contact with the medium the level of which has to be detected, characterized in that - between at least part of the overmoulded outer coating and the first electrodes there is set at least one intermediate insulation layer (20 3, 40, 40', 50; 20 3, 50; 20 3, 40, 40'; 40, 40', 50; 20 3; 40, 40'; 50) made of an electrically insulating material different from the first electrically insulating polymeric material, - the at least one intermediate insulation layer comprises a layer made of a material selected from among materials that include silicon, or derivatives or compounds thereof, and materials that include fluorine derivatives or compounds. - between at least part of the overmoulded outer coating and the first electrodes there is set at least one intermediate insulation layer (20 3, 40, 40', 50; 20 3, 50; 20 3, 40, 40'; 40, 40', 50; 20 3; 40, 40'; 50) made of an electrically insulating material different from the first electrically insulating polymeric material, - the at least one intermediate insulation layer comprises a layer made of a material selected from among materials that include silicon, or derivatives or compounds thereof, and materials that include fluorine derivatives or compounds.