Abstract: A fastener comprises a flange having an insertion hole, a plurality of circumferentially spaced clamping legs extending from the flange, and a nut connected integrally to ends of the clamping legs. The nut and the clamping legs are inserted into a mounting hole of a workpiece. Then the nut is pulled toward the flange to cause buckling of the clamping legs. The fastener is set to the workpiece by clamping the workpiece between the buckled clamping legs and the flange. Each of the clamping legs has a buckling portion, a nut-side leg portion and a flange-side leg portion. The buckling portion is thinner than the nut-side and flange-side leg portions. The nut-side leg portion has a higher strength than the flange-side leg portion. A region of the nut-side leg portion adjacent to the nut has a lower strength than that of the remaining region thereof.