To provide a thread structure capable of always providing sufficient tightening force.

This thread structure is achieved by forming a threaded hole 23 in a metallic plate 22, forming a notch 25 on part of an inner-periphery edge 24 of the threaded hole 23, the inner-periphery edge 24 being divided into segments 26, 27 by the notch 25 and setting an area ranging from the segment 26 to the segment 27 so as to have a spiral shape as it goes upward, thus permitting the spiral threaded hole inner-periphery edge 27 to have a circular shape when viewed from the vertical direction to the metallic plate 22.