Check valve (6) for medical infusion lines and the like, comprising a diaphragm made of elastic material set between a first tubular connector (2) and a second tubular connector (3) and constituted by the end wall (8) of a cup-shaped element (7), the outer peripheral edge (8a) of which is normally pressed in seal contact against an annular valve seat with conical surface (12) of the first tubular connector (2) under the axial thrust exerted by the side wall (9) of the cup-shaped element (7). The free edge (9a) of the side wall (9) of the cup-shaped element (7) is set resting against a transverse surface (14) of the second tubular connector (3) only in regions corresponding to angular portions (17) of the latter separated by non-resting angular portions (18).
