To preventing detachment of a screw with loosening prevention from a U-shaped joining part.

The joint implement is for joining wires having annular parts in at least one ends. It is provided with the U-shaped joining part 1 having an insertion hole 3, a screw part 4, and a U-shaped form, for inserting the annular parts of the wires, and an engagement screw 2 with loosening prevention movably engaged with the insertion hole 3 of the U-shaped joining part 1, having one end engaged with the screw part 4 of the U-shaped joining part 1, and carrying out loosening prevention of the inserted annular parts. In an inner side of the insertion hole 3 of the U-shaped joining part 1, the engagement screw 2 with loosening prevention has a loosening prevention structure including a welded detachment preventing part 20 of a shape larger than an opening. The structure can prevent detachment of a loosening prevention member from an insertion member.