A screw-fastening assembly having anti-loosening capabilities, comprising a first screw element and a second screw element adapted to be screw-fastened to the first screw element; wherein one of the first and second screw elements (11,12) includes an engagement recess R with an inner peripheral surface, the engagement recess being open toward the other screw element, the other screw element includes an engagement protrusion P, the engagement protrusion being introduced into the engagement recess R in a fastened state, the inner peripheral surface of the engagement recess R is formed in a cylindrical shape that is straight along an axial direction, the engagement protrusion P is shaped as a circular column or cylinder having a maximum-diameter portion at one location along the axial direction, the engagement protrusion P and the engagement recess R are eccentric relative to each other such that the maximum-diameter portion of the engagement protrusion P and the inner peripheral surface of the engagement recess R are in pressure contact only along part of their circumference in the fastened state, characterized in that the maximum -diameter portion of the engagement protrusion P is spaced apart from a base end of the engagement protrusion P by a predetermined distance so as to allow the engagement protrusion P to move forward and backward along the axial direction relative to the engagement recess R with the maximum-diameter portion remaining in pressure contact with the inner peripheral surface of the engagement recess only along part of their circumference, the maximum -diameter portion of the engagement protrusion is located close to a distal end of the engagement protrusion P, and the outer peripheral surface of the engagement protrusion P is a reverse-tapered surface having a diameter gradually increasing toward the distal end. wherein one of the first and second screw elements (11,12) includes an engagement recess R with an inner peripheral surface, the engagement recess being open toward the other screw element, the other screw element includes an engagement protrusion P, the engagement protrusion being introduced into the engagement recess R in a fastened state, the inner peripheral surface of the engagement recess R is formed in a cylindrical shape that is straight along an axial direction, the engagement protrusion P is shaped as a circular column or cylinder having a maximum-diameter portion at one location along the axial direction, the engagement protrusion P and the engagement recess R are eccentric relative to each other such that the maximum-diameter portion of the engagement protrusion P and the inner peripheral surface of the engagement recess R are in pressure contact only along part of their circumference in the fastened state, characterized in that the maximum -diameter portion of the engagement protrusion P is spaced apart from a base end of the engagement protrusion P by a predetermined distance so as to allow the engagement protrusion P to move forward and backward along the axial direction relative to the engagement recess R with the maximum-diameter portion remaining in pressure contact with the inner peripheral surface of the engagement recess only along part of their circumference, the maximum -diameter portion of the engagement protrusion is located close to a distal end of the engagement protrusion P, and the outer peripheral surface of the engagement protrusion P is a reverse-tapered surface having a diameter gradually increasing toward the distal end.