1. A joining method comprising:a butting process configured to butt a first metal member and a second metal member each having a front surface with various heights against each other to form therebetween a butt portion with varying heights, wherein the butt portion includes at least two flat sections, at least one upward inclined section and at least one downward inclined section; and
a welding process configured to apply a friction stirring to the butt portion by a stirring pin of a rotation tool, the stirring pin being maintained at a constant insertion depth with respect to at least one of the first and the second metal member across the flat sections and said upward and downward inclined sections of the butt portion,
wherein only the stirring pin contacts both the first metal member and the second metal member at the insertion depth generating only frictional heat and the stirring pin has a tapered shape so as to have a smaller diameter in a hanging down direction and includes only one outer circumferential surface having only one continuous spiral groove engraved only in the outer circumferential surface engaging the first metal member and the second metal member, and
wherein the spiral groove extends counterclockwise from a proximal end side to a distal end side of the outer circumferential surface of the stirring pin, while the rotation tool rotates clockwise, and the spiral groove extends clockwise from a proximal end side to a distal end side of the outer circumferential surface of the stirring pin, while the rotation tool rotates counterclockwise.