Drain valve for the treatment of glaucoma, surgically implantable in an eye of a patient, comprising: - a main body which accommodates a membrane valve device, and - a drainage tube connected at one end with said main body and the respective membrane valve device, wherein the main body is designed to be implanted by a surgeon on an outer surface of the eye globe, in an area under the conjunctiva, wherein the drainage tube is suitable to be positioned by the surgeon on the outer surface of the eye globe and to be implanted in the eye so as to penetrate, with its end (12f, 12f) opposite to that adjacent and connected with the main body, inside the anterior chamber of the eye globe, in order to drain and feed from the anterior chamber to the membrane valve device the aqueous humour, and wherein the membrane valve device, accommodated in the main body, is suitable for controlling the drainage, towards the outside, of the aqueous humour contained in said anterior chamber and fed by the drainage tube; the drain valve being characterised in that the drainage tube has a telescopic configuration comprising a first outer tube, connected at one end to the main body and the membrane valve device of the drain valve, and a second inner tube suitable for sliding and thereby to be extracted from the outer tube, so as to allow the surgeon to vary and adapt, during the surgery to implant the drain valve, the length of the drainage tube depending on the distance between the main body of the drain valve and the point wherein the drainage tube is to be implanted on the surface of the eye globe to penetrate inside the anterior chamber, in that the second inner tube has a tip portion consisting of an intraocular tube provided to be implanted in the eye so as to penetrate inside the anterior chamber of the eye globe, in that the intraocular tube is grafted at an angle of 135° on the remaining part, sliding in the first outer tube, of the second inner tube, and has a rigidity greater than that of the remaining part of the inner tube; in that the drainage tube comprises a gasket integral with the inner tube and interposed between the latter and the outer tube in order to ensure the sealing of the drainage tube against the outflow of aqueous humour, in that, in the relative sliding of the inner tube with respect to the outer tube, the gasket, integral with the inner tube, is suitable for abutting against a ridge or a narrowing (12a") formed at one end of the outer tube in order to limit the sliding and the stroke of the second inner tube with respect to the first outer tube; and in that the main body of the drain valve has a series of through holes having the function of placing in communication the soffit (S") of the main body, and thereby the surface of the eye globe on which the main body rests below, and the extrados (S') of the same main body, wherein the soffit (S"), of the main body of the valve, provided to rest and co-operate in contact with the outer surface of the eye globe, has at the base of and around each of the through holes, retaining bag suitable for receiving and retaining the aqueous humour, so as to improve the lubrication of the area of contact between the main body and the eye globe and therefore the adherence between these two parts, and wherein the extrados (S') of the main body of the drain valve has an opening, defined by a divergent and fan-shaped recess, suitable for facilitating both the flow and the evacuation of the aqueous humour from the inside towards the outside (S') of the main body and the channelling of the aqueous humour through said through holes towards the soffit (S") of the main body in contact with the surface of the eye globe, so as to facilitate and improve the lubrication of the eye globe and of the other parts of the eye in the area around the drain valve. - a main body which accommodates a membrane valve device, and - a drainage tube connected at one end with said main body and the respective membrane valve device,