1. A manufacturing apparatus for a robot arm including an arm main body with a long tubular or columnar shape having a uniform lateral cross-section, and attachment interface parts that are fixed to both sides of the arm main body and are attached to another member, the manufacturing apparatus comprising:a pair of fixing parts supporting at least a part of each of the attachment interface parts and each end of the arm main body in a positioned state and each including a space at least near a joint part between each of the attachment interface parts and the arm main body;
a slide mechanism capable of adjusting a distance between the two fixing parts along a longitudinal axis of the arm main body by moving at least one of the fixing parts; and
a material injection part that injects a molten material into at least a part of the space so as to integrally joint each of the attachment interface parts to each end of the arm main body,
wherein a material filling space, which is filled with the molten material, of the space has:
at least part of a first end proximate the arm main body, the first end having a cross sectional shape conforming to a cross sectional shape of the arm main body; and
at least a part of a second end proximate one of the attachment interface parts, the second end having a shape conforming to an outer peripheral surface of one of the attachment interface parts, and
wherein the material filling space has a hollow between at least the part of the first end and at least the part of the second end.