A strain wave gearing has three components and a temporary-fixing jig, three components being an internally toothed gear, a cup-shaped externally toothed gear, and a wave generator. The temporary-fixing jig is securely fastened to an output shaft fixed to the externally toothed gear by a temporary-fixing bolt and is securely fastened to an input shaft fixed to a cam plate of the wave generator by a temporary-fixing bolt. The temporary-fixing jig engages with the output shaft and the input shaft and maintains the three components in an assembled state. There is no need for an operation for adjusting the positions of the three components in an operation for attaching the strain wave gearing to a motor. After the strain wave gearing has been attached to the motor, the temporary-fixing jig is removed from the strain wave gearing, wherefore less space is required for installation.