To provide a welting tool to enable the decoration of edges at the sleeve opening and bottom opening of clothes and purse opening, etc., and give a function to constrict the opening, and applicable by freely adjusting the tool to the size of the sleeve opening, bottom opening and purse opening.

The welting tool 1 includes a flat bag belt 3 having a frill 2 on at least one edge, and through-holes 4 sporadically formed on one surface of the flat bag part 3 in the direction of length. The flat bag part 3 cut to a proper length is sewn to a purse opening 6, or sleeve opening 7a, bottom opening 7b, etc., of clothes 7 to decorate the opening. The opening can be nicely constricted by passing a string 5 through the flat bag part 3, extending both ends of the string 5 from two through-holes 4, 4 nearest to the end edges and tying the string 5.