PURPOSE: To prevent the seizing from being generated by making the circumferential distortion of the whole surface of a female screw being screwed in a valve which is the tensile distortion proportional to the valve tightening force, generating no axial distortion, and making the tensile distortion as main part even when the axial distortion is generated.

CONSTITUTION: The outer circumferential wall of a valve fitting screw part 2 corresponding to a lower side part at least in the axial direction of a female screw 3 is cut to make the wall thickness of this part smaller than that of the upper side part. When the relationship between the valve tightening torque at the screw part 2 and the circumferential and axial distortions is measured, the circumferential distortion is approximately increased in a linear manner in response to the increase of the valve tightening torque in any thread of the female screw 3. The distortion is not concentrated at one point, and the seizing with this concentrated point as the starting point can be prevented. The axial distortion is small compressive distortion at the third and eleventh threads, while tensile distortion is mainly generated at the first and eighth threads. This is also the cause to remove the seizing.