A press plate is tacked to a boss of a cup-shaped flexible externally toothed gear of a wave gear device unit. A second arm as a driven member is attached by a fastening bolt to an end surface on an outer side of an output flange of the wave gear device unit. The fastening bolt is screwed into and secured in a tap hole of the press plate from each bolt insertion hole of the second arm, via a flange-side bolt insertion hole and a boss-side bolt insertion hole. A fastening structure can be achieved in which a driven member is fastened to a wave gear device unit in which holes are easily machined into the output flange, and only a few fastening bolts are needed to fasten the boss, the output flange, and the driven member.