Method for manufacturing liquid-cooling jacket and friction stir welding method

The present invention includes: a primary joining process in which a coarse portion having a predetermined width is formed in the vicinity of a step side face within a plasticized region while the rotary tool is being moved one round along a first butted portion to perform friction stirring in a state that a tip of a stirring pin of a rotary tool being rotated is inserted to the same depth as or slightly deeper than a step bottom face and a bottom face of a shoulder portion is in contact with a front face of a sealing body and the stirring pin is slightly in contact with at least an upper portion of a jacket body; and an inspection process in which a passed position of the stirring pin is specified by performing, after the primary joining process, a flaw detection to detect the coarse portion.