For the purpose of achieving a further reduction in weight while maintaining strength and of reducing the manufacturing cost, the present invention provides a robot arm including: a long cylindrical arm body; and mounting interface portions that are fixed at both sides of the arm body and that are used to mount the robot arm to another member. At least sections of outer surfaces of the arm body and the mounting interface portions are formed of a resin. A first part that has one of the mounting interface portions and a section of the arm body and a second part that has the other one of the mounting interface portions and a section of the arm body are joined to each other.