To provide a rivet in which its fitting work and removal work after its use are facilitated, and a used state keeping capability is high; a lighting unit attachment/detachment prevention tool preventing an easy attachment/detachment work of an LED lamp and easily adapted for an existing lighting unit support device; and a lighting unit including the lighting unit attachment/detachment prevention tool.

A first member 3 has a head part 31 in which a surface 311 is a continuous stepless surface and an insertion part 33 protruded from a rear surface 312 of the head part 31. The second member 5 has a leg part 55 with an insertion hole 595 extending in one direction and flanges 51, 53 arranged at a side wall 551 enclosing the insertion hole 595 of the leg 55. The rivet 1 is constituted such that when the insertion part 33 of the first member 3 is inserted into the insertion hole 595 of the second member 5, an entire outer periphery of the head part 31 of the first member 3 is positioned within an area enclosed by the outer periphery of the flange 53 of the second member 5 as seen from its top plan view and the diameter of the leg part 55 of the second member 5 is expanded.