Provided is a joining method that can prevent a plastic flowing material from flowing out from a butt section and that can reduce the thickness and weight of metal members. The joining method is for joining a first metal member and a second metal member by using a rotary tool comprising a stirring pin, and is characterized in that: the stirring pin comprises a flat surface perpendicular to the rotation axis of the rotary tool and comprises a protruding section protruding from the flat face; and in a friction stirring step, the flat surface is brought into contact with the first metal member and the second metal member, and a front end face of the protruding section is inserted deeper than an upper overlapping section to join an upper front butt section and the upper overlapping section.