During endoscopic surgery, the operation accuracy can be improved if the endoscope system provides 3D information. In this paper, an adaptor lens is presented for a commercial 2D endoscope to generate stereoscopic images. The function of the adaptor lens is verified in this study, and we prove the adaptor lens can be compatible with an arbitrary rigid 2D endoscope system even if its original imaging lens specifications are unknown. The prototype adaptor lens kit, which was utilized to verify the basic idea, includes two commercial plastic lens modules. Furthermore, an optimized design of the adaptor lens system is also presented. The optical features of the optimized adaptor lens kit was analyzed via the ray-tracing software Zemax. The field of view (FOV) of the optimized adaptor lens system is 70°, and the dispersion of the optimized adaptor lens is small enough to be neglected.