Provided is a method for manufacturing a liquid-cooled jacket, to reduce the size of a recessed groove on a surface of a metal member and also to reduce roughness of a jointed surface. The method includes: a placing step of placing a sealing body on a jacket body, a first main joining step of performing friction stirring by moving a main joining rotary tool around to a first overlapped portion, and a second main joining step of performing friction stirring to a second overlapped portion. The main joining rotary tool has a base-end-side pin and a tip-end-side pin. A taper angle of the base-end-side pin is greater than a taper angle of the tip-end-side pin and a stairs-like pin step portion is formed on an outer circumferential surface of the base-end-side pin.