A method for manufacturing a liquid-cooled jacket (1, 1A, 101, 101A), comprising steps of: preparing to form a jacket body (2, 2A, 102, 102A) that has a bottom (10, 110) and a peripheral wall (11, 111) extending upward from a peripheral edge of the bottom (10, 110), as well as a seal body (3, 3A, 103, 103A) that has a first substrate (21, 121) formed of a first metal in a plate shape, a second substrate (22, 122) formed of a second metal in a plate shape on a front face (21a, 121a) of the first substrate (21, 121) so as to have a peripheral edge of the first substrate (21, 121) be exposed, and a plurality of fins (23, 123) arranged in parallel on a rear face (21b, 121b) of the first substrate (21, 121); arranging the seal body (3, 3A, 103, 103A) on the jacket body (2, 2A, 102, 102A) to form an overlaid portion with an end face (11a, 111a) of the peripheral wall (11, 111) and a rear face (21b, 121b) of the first substrate (21, 121) overlaid thereon; and joining by friction stir the overlaid portion by inserting a rotary tool from a front face (21a, 121a) of the peripheral edge of the first substrate (21, 121) and by moving the rotary tool along the overlaid portion relatively in a state where a stirring pin of the rotary tool is contacted with only the first substrate (21, 121) or with both the first substrate (21, 121) and the peripheral wall (11, 111), wherein in the preparing step, a support (12, 112) having a protrusion (14, 114) on an end face (12a, 112a) thereof is formed on the bottom (10, 110) of the jacket body (2, 2A, 102, 102A), wherein the first substrate (21, 121) is formed to have a hole (24, 124) therein and the second substrate (22, 122) is formed to have a vicinity of the hole (24, 124) be exposed on the front face (21a, 121a) of the first substrate (21, 121), wherein in the arranging step, the overlaid portion is formed and the protrusion (14, 114) is inserted in the hole (24, 124), 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 (14, 114) butted with a wall of the hole (24, 124). preparing to form a jacket body (2, 2A, 102, 102A) that has a bottom (10, 110) and a peripheral wall (11, 111) extending upward from a peripheral edge of the bottom (10, 110), as well as a seal body (3, 3A, 103, 103A) that has a first substrate (21, 121) formed of a first metal in a plate shape, a second substrate (22, 122) formed of a second metal in a plate shape on a front face (21a, 121a) of the first substrate (21, 121) so as to have a peripheral edge of the first substrate (21, 121) be exposed, and a plurality of fins (23, 123) arranged in parallel on a rear face (21b, 121b) of the first substrate (21, 121); arranging the seal body (3, 3A, 103, 103A) on the jacket body (2, 2A, 102, 102A) to form an overlaid portion with an end face (11a, 111a) of the peripheral wall (11, 111) and a rear face (21b, 121b) of the first substrate (21, 121) overlaid thereon; and joining by friction stir the overlaid portion by inserting a rotary tool from a front face (21a, 121a) of the peripheral edge of the first substrate (21, 121) and by moving the rotary tool along the overlaid portion relatively in a state where a stirring pin of the rotary tool is contacted with only the first substrate (21, 121) or with both the first substrate (21, 121) and the peripheral wall (11, 111), wherein in the preparing step, a support (12, 112) having a protrusion (14, 114) on an end face (12a, 112a) thereof is formed on the bottom (10, 110) of the jacket body (2, 2A, 102, 102A), wherein the first substrate (21, 121) is formed to have a hole (24, 124) therein and the second substrate (22, 122) is formed to have a vicinity of the hole (24, 124) be exposed on the front face (21a, 121a) of the first substrate (21, 121), wherein in the arranging step, the overlaid portion is formed and the protrusion (14, 114) is inserted in the hole (24, 124), 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 (14, 114) butted with a wall of the hole (24, 124).