We present a conceptual demonstration of a metallic planar lens composed of double-turn waveguides for sub-diffraction-limit focusing. The phase delay of a single double-turn waveguide dependent on its structural parameters is investigated by employing the finite-difference time-domain (FDTD) numerical method. The design utilizes the surface plasmon polaritons (SPPs) that propagate along the metal-insulator-metal (MIM) waveguides to achieve the desired spatial phase modulation in the transmitted field. The simulated focal length achieved is in positive agreement with the design and the full-width at half-maximum (FWHM) is 0.446λ, well beyond the diffraction limit. This superfocusing performance can be maintained very well under the slight change of film thickness and slit width, showing the robustness of the design. The maximum aspect ratio of nanoslits constructing the proposed lens is 3.33, which is far less than the previous reports, alleviating the later fabrication. The metallic planar lens as demonstrated will find its applications in such fields as lithography, integrated optics, and super-resolution imaging.