The method comprises a primary joining process in which primary joining is performed by friction stirring by moving a rotary tool (F) once around a recessed part (13) along a first overlap part (H1) in a state where only a stirring pin (F2) of the rotary tool provided with the stirring pin is inserted in the first overlap part (H1) from a front surface (3b) of a sealing body (3) and is in contact with a jacket body (2) and the sealing body (3). In the primary joining process, the rotary tool (F), which is provided with a flat surface (F4) orthogonal to a rotational axis of the stirring pin (F2) and a projection (F5) projecting from the flat surface (F4) at a tip part of the stirring pin (F2), is employed, and the first overlap part (H1) is joined by bringing the flat surface (F4) into contact with only the sealing body (3) and inserting a tip end of the projection (F5) more deeply than the first overlap part (H1).