To provide a tool for fastening a button which can smoothly fasten the button one-handed to the hard and small buttonhole of mainly a shirt of clothing, and can be employed for other uses.

In the tool for fastening the button which consists of a ring-shaped part 2 through which the button passes and the part of a tip groove 1, if the part of a groove 1 at the tip is inserted from the head of the buttonhole and the tool is drawn out by passing the target button from the part of the ring 2 of the tool, the button moves to the groove 1 at the tip, and if the tool is furthermore drawn out, the button can pass through the buttonhole. Moreover, the tool can be employed for untying the double knot end of the string and can also be employed for untying the square knot of the string.