Optical modes in subwavelength-scale nanostructures are hard to reach from conventional far-field optics because they mainly exist in the form of near-field. Here, we propose an experimental method that can map out the near-field optical modes of any arbitrary nanostructures. We set up a far- to near-field transmission matrix system by using near-field scanning optical microscope and wavefront shaping of incident wave. By applying the singular value decomposition of the measured transmission matrix, we could identify symmetric and antisymmetric modes of a pair of nano-antenna whose width and separation are well below the diffraction limit. Our method will help designing complex functional nanostructures by providing the experimental means of understanding their optical response.