A method for manufacturing a liquid-cooling jacket (1) where heat transfer fluid flows in a hollow part (14) defined by a jacket body (2) and a sealing body (3) includes: an overlapping process in which the sealing body (3) is placed on an end surface (11a) of a peripheral wall part (11) in such a way that the end surface (11a) and a back surface of the sealing body (3) are overlapped each other to form a first overlapped part (H1); and a primary joining process in which primary joining is performed by friction stirring in such a way that a rotary tool (FD) is moved once around a recessed part (13) along the first overlapped part (H1). In the primary joining process, in a state where a base side pin of the rotary tool (FD) is in contact with the sealing body (3), a flat surface of the base (tip) side pin is brought in contact with only the sealing body (3), and a tip of a projection projecting from the flat surface is inserted more deeply than the first overlapped part (H1) to join the first overlapped part (H1).