A method for manufacturing a liquid-cooled jacket , comprising steps of: preparing to form a jacket body that has a bottom and a peripheral wall extending upward from a peripheral edge of the bottom , as well as a seal body that has a first substrate formed of a first metal in a plate shape, a second substrate formed of a second metal in a plate shape on a front face of the first substrate so as to have a peripheral edge of the first substrate be exposed, and a plurality of fins arranged in parallel on a rear face of the first substrate ; arranging the seal body on the jacket body to form an overlaid portion (J1) with an end face of the peripheral wall and a rear face of the first substrate overlaid thereon; and joining by friction stir the overlaid portion (J1) by inserting a rotary tool (F) from a front face of the peripheral edge of the first substrate and by moving the rotary tool (F) along the overlaid portion (J1) relatively in a state where a stirring pin (F2) of the rotary tool (F) is contacted with only the first substrate or with both the first substrate and the peripheral wall , wherein in the preparing step, a support having a protrusion on an end face thereof is formed on the bottom of the jacket body , wherein the first substrate is formed to have a hole therein and the second substrate is formed to have a vicinity of the hole be exposed on the front face of the first substrate , wherein in the arranging step, the overlaid portion (J1) is formed and the protrusion is inserted in the hole , and wherein in the joining step, friction stir joining is applied to a butted portion (J2, J12) that is formed with an outer periphery of the protrusion butted with a wall of the hole . preparing to form a jacket body that has a bottom and a peripheral wall extending upward from a peripheral edge of the bottom , as well as a seal body that has a first substrate formed of a first metal in a plate shape, a second substrate formed of a second metal in a plate shape on a front face of the first substrate so as to have a peripheral edge of the first substrate be exposed, and a plurality of fins arranged in parallel on a rear face of the first substrate ; arranging the seal body on the jacket body to form an overlaid portion (J1) with an end face of the peripheral wall and a rear face of the first substrate overlaid thereon; and joining by friction stir the overlaid portion (J1) by inserting a rotary tool (F) from a front face of the peripheral edge of the first substrate and by moving the rotary tool (F) along the overlaid portion (J1) relatively in a state where a stirring pin (F2) of the rotary tool (F) is contacted with only the first substrate or with both the first substrate and the peripheral wall , wherein in the preparing step, a support having a protrusion on an end face thereof is formed on the bottom of the jacket body , wherein the first substrate is formed to have a hole therein and the second substrate is formed to have a vicinity of the hole be exposed on the front face of the first substrate , wherein in the arranging step, the overlaid portion (J1) is formed and the protrusion is inserted in the hole , and wherein in the joining step, friction stir joining is applied to a butted portion (J2, J12) that is formed with an outer periphery of the protrusion butted with a wall of the hole . <EOS>