A method of joining a first metal member and a second metal member made of different materials by using a rotary tool provided with a stirring pin having a diameter becoming smaller toward an extremity of the stirring pin, wherein the first metal member is made of aluminum or an aluminum alloy, and the second metal member is made of copper or a copper alloy, the method being characterized by the following steps: a preparatory step of preparing the first metal member having a first upper surface, a first lower surface, and a first inclined surface connecting the first upper surface and the first lower surface, and the second metal member having a second upper surface, a second lower surface, and a second inclined surface connecting the second upper surface and the second lower surface, the second metal member having a higher melting temperature than the first metal member; a butting step of forming a butting interface by butting the first inclined surface and the second inclined surface against each other in such a manner that a first upper intersecting line formed by the first upper surface and the first inclined surface is positioned further in a direction of the second metal member than a first lower intersecting line formed by the first lower surface and the first inclined surface, and a second lower intersecting line formed by the second lower surface and the second inclined surface is positioned further in a direction of the first metal member than a second upper intersecting line formed by the second upper surface and the second inclined surface; and a joining step of joining the first metal member and the second metal member, wherein in the joining step, the first metal member and the second metal member are joined by moving the rotary tool along the butting interface while the rotary tool being rotated is inserted into the first metal member from only the first upper surface and only the stirring pin is in contact with only the first metal member, and a rotational direction and a moving direction of the rotary tool are set so that a second metal member side of a plasticized region to be formed along a moving locus of the rotary tool is a shear side, and a first metal member side of the plasticized region is a flow side, wherein the shear side is a side where a relative speed of the outer peripheral surface of the rotary tool to the joining portion is a speed obtained by adding a feed speed and a tangential speed of the outer peripheral surface of the rotary tool, and wherein the flow side is a side where a relative speed of the outer peripheral surface of the rotary tool to the joining portion is a speed obtained by subtracting the feed speed and the tangential speed of the outer peripheral surface of the rotary tool. a preparatory step of preparing the first metal member having a first upper surface, a first lower surface, and a first inclined surface connecting the first upper surface and the first lower surface, and the second metal member having a second upper surface, a second lower surface, and a second inclined surface connecting the second upper surface and the second lower surface, the second metal member having a higher melting temperature than the first metal member; the first metal member having a first upper surface, a first lower surface, and a first inclined surface connecting the first upper surface and the first lower surface, and the second metal member having a second upper surface, a second lower surface, and a second inclined surface connecting the second upper surface and the second lower surface, the second metal member having a higher melting temperature than the first metal member; a butting step of forming a butting interface by butting the first inclined surface and the second inclined surface against each other in such a manner that a first upper intersecting line formed by the first upper surface and the first inclined surface is positioned further in a direction of the second metal member than a first lower intersecting line formed by the first lower surface and the first inclined surface, and a second lower intersecting line formed by the second lower surface and the second inclined surface is positioned further in a direction of the first metal member than a second upper intersecting line formed by the second upper surface and the second inclined surface; and a joining step of joining the first metal member and the second metal member, wherein in the joining step, the first metal member and the second metal member are joined by moving the rotary tool along the butting interface while the rotary tool being rotated is inserted into the first metal member from only the first upper surface and only the stirring pin is in contact with only the first metal member, and a rotational direction and a moving direction of the rotary tool are set so that a second metal member side of a plasticized region to be formed along a moving locus of the rotary tool is a shear side, and a first metal member side of the plasticized region is a flow side, wherein the shear side is a side where a relative speed of the outer peripheral surface of the rotary tool to the joining portion is a speed obtained by adding a feed speed and a tangential speed of the outer peripheral surface of the rotary tool, and wherein the flow side is a side where a relative speed of the outer peripheral surface of the rotary tool to the joining portion is a speed obtained by subtracting the feed speed and the tangential speed of the outer peripheral surface of the rotary tool.