To provide a fixture improving a reliability of thread connection using an irreversible locking means to prevent looseness between a bolt and a nut completely with a small force.

The fixture comprises a bolt 1 and a nut 2 screwed with the bolt, the bolt 1 includes a head 10 adapted to a turning implement in the upper section, and a male screw portion 11 in the lower section. Part of the male screw portion includes at least two or more screw thread removed portion in the longitudinal direction of the bolt. The nut 2 includes a base 20 in the upper section having an outer shape adapted to the turning implement and a female screw 23 to be engaged with the male screw portion of the bolt 1 inside and a tubular crushing portion 21 in the lower section having an inner diameter larger than an inner circumference of the female screw 23. After screwing the bolt/nut at a predetermined location, a pressure is applied to the side of the crushing tube portion 21 in a beltlike pattern toward the screw thread removed portion 12 of the bolt 1, and the side of the nut crushing tube portion 21 is crushed to the screw thread removed portion 12.

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