Sealing device for a space valve or a fitting used in aeronautics or aerospace, comprising a vertical axis (OX), a horizontal axis (OY), an axis (OZ), a first metal part (A) having a first sealing zone which has a depth (PA), a mixed part (B) consisting of at least two parts: - a second metal part and - a PTFE pellet comprising a second sealing zone which comprises a recess with a shape matching the shape of the first sealing zone and with a depth corresponding to that of the first sealing zone , the first sealing zone and the second sealing zone have complementary male-female type shapes allowing them to fit into one another to allow the pellet to perfectly match the contact surface of the first sealing zone , characterized in that : - the first sealing zone consists of a flat surface having a width (LP) and of at least two fillets having a radius R1 and R2, - the pellet is held on the metal part by a metal ring to limit the mobility of said pellet and to keep said pellet substantially stable under the effect of mechanical and/or vibratory stresses, and - said device further comprises at least one elastic membrane with a triple function, said elastic membrane being elastically pre-deformed in the initial state because it is mounted on a metal spacer at an angle (ALFA) such that the point of attachment of said membrane on said second metal part is at a height (DE) making it possible to obtain a preload (P) resulting from the deformation caused by a displacement of the membrane from said height (DE) along the vertical axis (OX) and from said angle (ALFA) less than 60 degrees, said preload (P) being proportional to said angle (ALFA) between the first metal part (A) and the mixed part (B) and greater than or equal to a contact force (EC) corresponding to the sum of the external forces liable to break the seal and/or accidentally separate said sealing zones and of parts (A) and (B) respectively. - a second metal part and - a PTFE pellet comprising a second sealing zone which comprises a recess with a shape matching the shape of the first sealing zone and with a depth corresponding to that of the first sealing zone , - the first sealing zone consists of a flat surface having a width (LP) and of at least two fillets having a radius R1 and R2, - the pellet is held on the metal part by a metal ring to limit the mobility of said pellet and to keep said pellet substantially stable under the effect of mechanical and/or vibratory stresses, and - said device further comprises at least one elastic membrane with a triple function, said elastic membrane being elastically pre-deformed in the initial state because it is mounted on a metal spacer at an angle (ALFA) such that the point of attachment of said membrane on said second metal part is at a height (DE) making it possible to obtain a preload (P) resulting from the deformation caused by a displacement of the membrane from said height (DE) along the vertical axis (OX) and from said angle (ALFA) less than 60 degrees, said preload (P) being proportional to said angle (ALFA) between the first metal part (A) and the mixed part (B) and greater than or equal to a contact force (EC) corresponding to the sum of the external forces liable to break the seal and/or accidentally separate said sealing zones and of parts (A) and (B) respectively. <EOS>