To prevent the occurrence of looseness by the co-rotation of an insert 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 hard board and the like.

Threads 4 are formed around the periphery of a shaft body 1 having a screw hole 2 in its head part. Projected parts 8 are arranged among the adjacent crests of the threads 4. The projected parts 8 are circumferentially inclined from the preceding side end of a rotational direction toward the following end side thereof when the projected parts 8 are screwed, and a rear side end surface 9 is formed into an approximately perpendicular surface. The insert bolt can be prevented from being rotated in a loosening direction by the engagement of the end surface 9 with the internal texture of a hard board and the like, and the occurrence of looseness can be prevented, which is caused by the co-rotation of the insert bolt when the bolt threadedly engaged with the screw hole 2 is unscrewed.