A joining method for joining a first metal member and a second metal member together using a rotary tool (F1D) for friction stirring comprising a base side pin and a tip side pin, comprising: a preparation process of forming a first step bottom face (15a, 16a) and a first step side face (15b, 16b) on a top face (11b, 12b) of an end portion of the first metal member and forming a second step bottom face (25a, 26) and a second step side face (25b, 26b) on a back face (21c, 22c) of an end portion of the second metal member; an overlapping and abutting process of forming an overlapped portion (J1a, J2a) of the first step bottom face (15a, 16a) and the second step bottom face (25a, 26a) which are overlapped with each other and forming a top side abutted portion (J1b, J2b) of the first step side face (15b, 16b) and an end face (21a, 22a) of the second metal member which abut each other and a back side abutted portion (J1c, J2c) of the second step side face (25b, 26b) and an end face (11a, 12a) of the first metal member which abut each other; and a friction stirring process of inserting the tip side pin that is rotating into the top side abutted portion (J1b, J2b) and moving the rotary tool (F1D) along the top side abutted portion (J1b, J2b) with each of the base side pin and the tip side pin being in contact with both the first metal member and the second metal member to join each of the top side abutted portion (J1b, J2b) and the overlapped portion (J1a, J2a), wherein the base side pin has a first taper angle while the tip side pin has a second taper angle that is smaller than the first taper angle, a circumferential face of the base side pin has a step portion being a spiral shape in plan view and a staircase shape in cross-sectional view formed thereon, wherein the step portion includes a step bottom face and a step side face, wherein the step bottom face slopes in the direction of the circumference from the rotation axis of the rotary tool (F1; F1D) to the circumference at an angle within -5° to 15° relative to the horizontal face, wherein a step angle between the step bottom face and the step side face is 85° to 120°, wherein the tip side pin has a flat face that is vertical to a rotation axis of the rotary tool (F1D) and a protruding portion protruding from the flat face, and wherein each of the first metal member and the second metal member is made of aluminium or aluminium alloys, and wherein each of the top side abutted portion (J1b, J2b) and the overlapped portion (J1a, J2a) is being joined with the flat face being in contact with both the first metal member and the second metal member and with a tip face of the protruding portion being inserted deeper than the overlapped portion (J1a, J2a) in the friction stirring process. a preparation process of forming a first step bottom face (15a, 16a) and a first step side face (15b, 16b) on a top face (11b, 12b) of an end portion of the first metal member and forming a second step bottom face (25a, 26) and a second step side face (25b, 26b) on a back face (21c, 22c) of an end portion of the second metal member; an overlapping and abutting process of forming an overlapped portion (J1a, J2a) of the first step bottom face (15a, 16a) and the second step bottom face (25a, 26a) which are overlapped with each other and forming a top side abutted portion (J1b, J2b) of the first step side face (15b, 16b) and an end face (21a, 22a) of the second metal member which abut each other and a back side abutted portion (J1c, J2c) of the second step side face (25b, 26b) and an end face (11a, 12a) of the first metal member which abut each other; and a friction stirring process of inserting the tip side pin that is rotating into the top side abutted portion (J1b, J2b) and moving the rotary tool (F1D) along the top side abutted portion (J1b, J2b) with each of the base side pin and the tip side pin being in contact with both the first metal member and the second metal member to join each of the top side abutted portion (J1b, J2b) and the overlapped portion (J1a, J2a), wherein the base side pin has a first taper angle while the tip side pin has a second taper angle that is smaller than the first taper angle, a circumferential face of the base side pin has a step portion being a spiral shape in plan view and a staircase shape in cross-sectional view formed thereon, wherein the step portion includes a step bottom face and a step side face, wherein the step bottom face slopes in the direction of the circumference from the rotation axis of the rotary tool (F1; F1D) to the circumference at an angle within -5° to 15° relative to the horizontal face, wherein a step angle between the step bottom face and the step side face is 85° to 120°, wherein the tip side pin has a flat face that is vertical to a rotation axis of the rotary tool (F1D) and a protruding portion protruding from the flat face, and wherein each of the first metal member and the second metal member is made of aluminium or aluminium alloys, and wherein each of the top side abutted portion (J1b, J2b) and the overlapped portion (J1a, J2a) is being joined with the flat face being in contact with both the first metal member and the second metal member and with a tip face of the protruding portion being inserted deeper than the overlapped portion (J1a, J2a) in the friction stirring process.