It is an object to provide a friction stir tool excellent in productivity, high temperature strength, and wear resistance at high temperatures. The friction stir tool is formed of a Co-based alloy comprising crystal grains containing a [gamma]' precipitate phase dispersed and precipitated therein, and a crystal grain boundary region and a precipitate phase between adjacent crystal grains, in which the precipitate phase is at least one phase selected from a [mu] phase, a Laves phase and a carbide phase.