To provide a fastening part using a screw, having very large locking force.

One or two recessed parts CA1 are formed on one side surface S of a male screw 10, a part of a locking body 20 including a flexible linear part L fitted in the recessed part CA1 parallel to the axis Ax of the male screw 10 is fastened by the male screw, and a part of a recessed surface P1 forming the above recessed part CA1 and parallel to the axis Ax of the male screw 10 is extended outward in the direction FA of fastening the male screw 10 from the inside of the valley of the male screw 10, and intersects the side surface S at an acute angle through the right inside of the valley of the male screw 10 on a plane Pa including a linear part L of the locking body 20 and the axis Ax of the male screw 10.