To provide an inexpensive drop-out preventive structure of a nut capable of easily preventing dropping-out of the nut with at a simple structure.

A male screw 10a of a tip of a screw portion of a bolt 10 is set up in dimension so that a root diameter d1 of the male screw 10a is larger than an inner diameter D2 corresponding to a female screw 12a of the nut 12 engaged with the bolt 10. Thus, the male screw 10a of the tip of the screw portion is formed in a slightly different shape causing tighter engagement with the nut when compared with a root diameter d2 of a male screw 10b at a base end side of a screw portion as a regular standard dimension corresponding to a female screw 12a of the nut 12.