A circle of planar nanoholes in a dielectric thin film can focus light into a sub-half-wavelength spot. The nanostructure with higher rotational symmetry shows better focusing properties. From finite-difference time-domain calculations, we verified such a focusing spot coming from the constructive interference of diffraction beams near the nanoholes. For a $1\mu \mathrm{m}$ size lens with eight $200\mathrm{nm}$ diameter holes, we achieved an optical spot smaller than $250\mathrm{nm}$ measured at $650\mathrm{nm}$ wavelength. This nanostructure provides a simple way to massively fabricate a planar lens array with a scale down to the submicrometer level.