An edge of each of a first metal member and a second metal member is formed to have a U-shaped cross section with a first horizontal plate projecting from a base edge of a vertical plate, and with a second horizontal plate projecting from a top edge of the vertical plate. The second metal member includes a projecting part. The joining method includes: an abutting step of forming an abutted part by abutting a front surface of the first horizontal plate the first metal member against a second side surface of the second metal member; and a joining step of performing friction stir welding of the first metal member and the second metal member by inserting a stirring pin from the projecting part and moving a joining rotating tool along an inner corner at the top edge of the vertical plate of the second metal member while making only the stirring pin in contact with the first metal member and the second metal member.