A joining method by which a pair of metal members are joined to each other with use of a rotary tool provided with a stirring pin having a smaller diameter toward a tip thereof, the method comprising: a preparation process in which a first metal member made of an aluminum alloy, a second metal member made of an aluminum alloy, and an auxiliary member made of aluminum or an aluminum alloy having a hardness lower than those of the first metal member and the second metal member and provided with an inclined face on at least one of a side face and another side face in such a form that the auxiliary member has a smaller dimension with increasing distance from the front face, and at least one of an end face of an end portion of the first metal member and an end face of an end portion of the second metal member is provided with an inclined face which corresponds to the inclined face of the auxiliary member and is inclined from a front face toward a back face are prepared; a butting process in which butted portions (J1, J2) are formed in such a form that the end portion of the first metal member and the end portion of the second metal member face each other, the auxiliary member is interposed in a gap formed between the end portions, the end face of the first metal member is butted against one of the side face of the auxiliary member, and the end face of the second metal member is butted against the other side face of the auxiliary member; and a joining process in which the first metal member and the second metal member are joined with the auxiliary member interposed therebetween by moving the rotary tool along the butted portions (J1, J2) in a state where the rotary tool which is being rotated is inserted only from a front face side of the auxiliary member, only the stirring pin is in contact with the auxiliary member, a base end side of the stirring pin is exposed, and an outer circumferential face of the stirring pin is slightly in contact with the first metal member and the second metal member. a preparation process in which a first metal member made of an aluminum alloy, a second metal member made of an aluminum alloy, and an auxiliary member made of aluminum or an aluminum alloy having a hardness lower than those of the first metal member and the second metal member and provided with an inclined face on at least one of a side face and another side face in such a form that the auxiliary member has a smaller dimension with increasing distance from the front face, and at least one of an end face of an end portion of the first metal member and an end face of an end portion of the second metal member is provided with an inclined face which corresponds to the inclined face of the auxiliary member and is inclined from a front face toward a back face are prepared; a butting process in which butted portions (J1, J2) are formed in such a form that the end portion of the first metal member and the end portion of the second metal member face each other, the auxiliary member is interposed in a gap formed between the end portions, the end face of the first metal member is butted against one of the side face of the auxiliary member, and the end face of the second metal member is butted against the other side face of the auxiliary member; and a joining process in which the first metal member and the second metal member are joined with the auxiliary member interposed therebetween by moving the rotary tool along the butted portions (J1, J2) in a state where the rotary tool which is being rotated is inserted only from a front face side of the auxiliary member, only the stirring pin is in contact with the auxiliary member, a base end side of the stirring pin is exposed, and an outer circumferential face of the stirring pin is slightly in contact with the first metal member and the second metal member.