A stringer manufacturing method for machining one end portion of an elongated member that is provided with a cap flange, a web, and a base flange and that has an inverted T-shape in a front view into a desired shape to obtain a desired stringer (22;32), wherein the one end portion of the elongated member is machined into a desired shape by using a vertical articulated robot that has at least 6 axes and that has, at the tip of an arm, an abrasive nozzle assembly that injects ultrahigh-pressure water containing abrasive and a catcher cup that recovers the ultrahigh-pressure water injected from the abrasive nozzle assembly, the stringer manufacturing method comprising the steps of: disposing the abrasive nozzle assembly and the catcher cup at one lateral side of the elongated member such that the abrasive nozzle assembly is located higher than the cap flange and the catcher cup is located lower than the cap flange, and disposing the abrasive nozzle assembly so as to be located closer to the central axis of the elongated member than the catcher cup is, and cutting off a one-lateral-side portion of the cap flange and an upper end portion of the web that are located at the one end portion of the elongated member; disposing the abrasive nozzle assembly and the catcher cup at the other lateral side of the elongated member such that the abrasive nozzle assembly is located higher than the cap flange and the catcher cup is located lower than the cap flange, and disposing the abrasive nozzle assembly so as to be located closer to the central axis of the elongated member than the catcher cup is, and cutting off an other-lateral-side portion of the cap flange and an upper end portion of the web that are located at the one end portion of the elongated member; and locating the catcher cup at one lateral side of the web, after locating the abrasive nozzle assembly at the other lateral side of the web, and disposing the abrasive nozzle assembly and the catcher cup such that the central axis thereof is parallel to an upper surface of the cap flange and a lower surface of the base flange, and cutting off a portion of the web that is located at the one end portion of the elongated member. disposing the abrasive nozzle assembly and the catcher cup at one lateral side of the elongated member such that the abrasive nozzle assembly is located higher than the cap flange and the catcher cup is located lower than the cap flange, and disposing the abrasive nozzle assembly so as to be located closer to the central axis of the elongated member than the catcher cup is, and cutting off a one-lateral-side portion of the cap flange and an upper end portion of the web that are located at the one end portion of the elongated member; disposing the abrasive nozzle assembly and the catcher cup at the other lateral side of the elongated member such that the abrasive nozzle assembly is located higher than the cap flange and the catcher cup is located lower than the cap flange, and disposing the abrasive nozzle assembly so as to be located closer to the central axis of the elongated member than the catcher cup is, and cutting off an other-lateral-side portion of the cap flange and an upper end portion of the web that are located at the one end portion of the elongated member; and locating the catcher cup at one lateral side of the web, after locating the abrasive nozzle assembly at the other lateral side of the web, and disposing the abrasive nozzle assembly and the catcher cup such that the central axis thereof is parallel to an upper surface of the cap flange and a lower surface of the base flange, and cutting off a portion of the web that is located at the one end portion of the elongated member.