Provided is a method of producing a liquid-cooled jacket wherein deformation of a seal body can be minimized. A method of producing a liquid-cooled jacket constituted by fixing a seal body which seals the opening of a recess, by friction stir welding, to a jacket body having a partially opening recess through which heat transport fluid for transporting heat generated by a heat generating body to the outside flows, wherein the seal body is mounted on a supporting surface which is formed at the peripheral edge of the opening of the recess in the jacket body and consists of the bottom surface of a step lower than the surface of the jacket body, the side surface of the step of the jacket body and the outer circumferential surface of the seal body are butted each other, a plasticized region is formed by making one round of a rotary tool which is equipped with a stir pin having a length greater than the thickness of the seal body round along the butting portion of the side surface of the step of the jacket body and the outer circumferential surface of the seal body, and then the seal body is welded to the jacket body.