Abstract: A check valve having a predominantly cylindrical housing, a sealing diaphragm comprising a thin, tubular slip and at one extremity a radially outwardly extending fastening flange, said flange being clamped between an uninterrupted inner shoulder in the housing and an annular locking member of a synthetic resin, having an outer surface being at least partly tapering, said locking member being enclosed between the fastening flange and an inwardly projecting protuberance of the housing, and a rotation symmetrical closing member being axially displaceable between a closing position sealing on the slip and an opened position away from said slip.When assembling the component parts of this check valve the diaphragm is first installed and thereafter the locking member is forced in position by an axial movement only.