Abstract: A screw coupling joint having a sealing portion between an outer circumference formed in convex in an axial direction of the coupling joint at the end of the male screw portion and an inner circumference formed in taper at an inner part of the female screw portion. The female screw portion has a stopper end positioned to be contacted, when the coupling joint is assembled, by the inner end of the male screw portion. The taper is between 1.degree. and 3.degree. and the inner diameter D.sub.1 of the female screw and the outer diameter D.sub.2 of the male screw have the relationship D.sub.2 >D.sub.1 at a tangent point therebetween. The end of the male screw portion and the end of the stopper are vertical or perpendicular to the axis of the coupling joint.

Abstract: A basic structure of a screw joint coupling for oil pipes according to the invention is that a main sealing portion is constructed with a sealing portion formed axially in convex at an end of a male screw, and with a sealing portion formed in taper at an inner side of a female screw, and that the male screw and the female screw are butted at an end point of the former and an end part of a stopper formed at an inner side of the latter. The main sealing portion has a shrinkage allowance .DELTA.D of the seal part, which is regulated in size in a relation between a shrinkage allowance .DELTA.d of the screw and a pressure P of the sealing part. In addition, specific values are selected as to the load flank angle, the radius of curvature in convex of the sealing portion of the male screw, and the tapering angle of the sealing portion of the female screw.