A strain wave gearing (1) has a flexible externally toothed gear (3) with a boss (7). A boss-side fastening surface (16) formed on the boss is coaxially superposed with a member-side fastening surface (15) of an output member (11), and the boss (7) and the output member are fastened by fastening bolts (13). The bolt tension of the fastening bolts (13) causes the engaging protuberances (20) of the boss-side fastening surface (16) to dig into the member-side fastening surface (15) by a predetermined dig-in depth. A large frictional force is produced in the fastening portions between the flexible externally toothed gear (3) and the output member (11). It is thereby possible to transmit the necessary torque by the bolt tension alone without using pins or a friction sheet together with the fastening bolts.