To eliminate such a fault that a nut slips to drop down from a hand or a globe in screwing a nut to a male screw part of a bolt with the nut grasped with finger tips when fastening the nut to the bolt with a working tool.

A structure to prevent falling of a bolt and a nut includes: a bolt having a male screw part 5 protruding from a head 3, a non-male screw part 6 protruding from the male screw part and having a diameter smaller than the male screw part, and a dropout prevention part 10 integrated with a tip of the non-male screw part to block nut dropout with the nut held in the non-male screw part while allowing the nut to pass through; and a nut 20 having a female screw part 21 screwable to the male screw part and having an inner diameter to allow the bolt to pass through the outer circumference of the dropout prevention part. The dropout prevention part has at least one small convexed part 12 protruding in a direction of the outer radius from an outer circumferential surface of a base 10a integrated with the non-male screw part, wherein a small concaved part 22 passing through the small convexed part is formed in an inner circumferential surface of the nut.