A joint structure comprising: a first metallic material having a first projection; a second metallic material similar in type to the first metallic material and weldable to the first metallic material, the second metallic material having a first penetrating part larger in diameter or width than the first projection, the first projection being positioned in the first penetrating part; and a different type of material different in type from the first metallic material and the second metallic material and difficult to be welded to the first metallic material and the second metallic material, the different type of material having a second penetrating part larger in diameter or width than the first projection, a rim of the second penetrating part being spaced radially or widthwise from the first projection by a first gap, wherein the first projection is positioned in the second penetrating part with the first gap between the first projection and the second penetrating part,, and the first projection is further positioned in the first penetrating part so that the different type of material is sandwiched between the first metallic material and the second metallic material, and an outer edge of the first projection of the first metallic material is welded to an inner edge of the first penetrating part so that the first metallic material and the second metallic material are melted and joined together to compress and fix the different type of material between the first metallic material and the second metallic material, thereby fixing the first metallic material, the second metallic material, and the different type of material together, characterized in that when the different type of material is sandwiched between the first metallic material and the second metallic material, the first metallic material and the second metallic material are melted and joined together with an end of the first projection extending further than the first penetrating part. a first metallic material having a first projection; a second metallic material similar in type to the first metallic material and weldable to the first metallic material, the second metallic material having a first penetrating part larger in diameter or width than the first projection, the first projection being positioned in the first penetrating part; and a different type of material different in type from the first metallic material and the second metallic material and difficult to be welded to the first metallic material and the second metallic material, the different type of material having a second penetrating part larger in diameter or width than the first projection, a rim of the second penetrating part being spaced radially or widthwise from the first projection by a first gap, wherein the first projection is positioned in the second penetrating part with the first gap between the first projection and the second penetrating part,, and the first projection is further positioned in the first penetrating part so that the different type of material is sandwiched between the first metallic material and the second metallic material, and an outer edge of the first projection of the first metallic material is welded to an inner edge of the first penetrating part so that the first metallic material and the second metallic material are melted and joined together to compress and fix the different type of material between the first metallic material and the second metallic material, thereby fixing the first metallic material, the second metallic material, and the different type of material together, characterized in that when the different type of material is sandwiched between the first metallic material and the second metallic material, the first metallic material and the second metallic material are melted and joined together with an end of the first projection extending further than the first penetrating part.