An auxiliary device for measuring the coaxiality of a lens. The auxiliary device includes a cylindrical main body having a first surface and a flat second surface facing away the first surface, and a protrusion formed on the first surface and defining a reference convex surface. The second surface of the main body defines a position part. The position part is coaxial with the reference convex surface and configured to engage with the lens to measure the coaxiality of the lens.