A method for producing an arm structure in which the arm structure is produced by: forming an arm precursor member having an external shape of the arm structure by, in a state in which a die is closed with a metal pipe member being disposed in a cavity thereof, pressurizing the pipe member with liquid supplied to an inside thereof to cause an external surface of the thus expanded pipe member to be pressed against an inner surface of the cavity; and forming a flange portion to be attached to a driven body by machining at least an end of the formed arm precursor member.