To prevent occurrence of looseness, caused by the co-rotation of an inert bolt when a bolt or a nut screwed in the insert bolt is unscrewed under a condition that the insert bolt is attached to a soft board and the like.

Threads 4 are formed around the periphery of a shaft body 1 which has a screw hole 2 in its head part and whose this is narrowed. Projected pieces 5 deformed by resistance at the time of screwing are arranged among the adjacent threads 4. In screwing when this insert bolt is screwed into a soft board, the projected pieces 5 are inclined to the following side of a screwing direction, and rotation in the loosening direction of the insert bolt is prevented by the engaging force of the projected pieces 5 inclined.