Threaded fastener, and function addition member of threaded fastener

A screwing fastener, comprising:

• a fastener main body having a seat face;
• a screw portion for tightening the fastener main body to a fastened member;
• a flange portion provided to the fastener main body, the flange portion is provided with a flange main body and an annular protruding portion that protrudes outward from an outer circumference of the flange main body;
• and
• a function adding member that is mounted to the flange portion, the screwing fastener configured to be tightened with the function adding member configured to be compressed between the flange portion and the fastened member,
• wherein the function adding member has at least a two-layer structure of a first adding portion that includes a recessed portion configured to be fit with the annular protruding portion and having slidability, and a second adding portion that is configured to come into contact with the fastened member,
• wherein the first adding portion is rotationally slidably in contact with the flange portion.