Plasmonic nanoantennas provide powerful tools in enhancing light−matter interactions by linking propagating far fields with localized near fields. Microscopic understanding on the enhancements of light−matter interactions requires precise knowledges on the near-field distributions upon nanoantenna excitations. In this article, we study nearfield distributions of nanoantennas under oblique illumination. We acquire the amplitude-and phase-resolved images of the near fields on Au nanorods in the infrared range, by use of the scattering-type scanning nearfield optical microscopy and the interferometric homodyne detection. Asymmetric near-field distributions are experimentally visualized and theoretically reproduced. The asymmetry in the near-field distributions is attributed to the preferential excitation of the surface plasmon polariton in one direction and the finite loss of the surface plasmon polariton inside the cavity. Furthermore, we organize the excitation conditions of the nanoantenna modes under oblique illumination as two formulas, each of which illustrates the cavity resonance condition and the phase matching condition for surface plasmon polariton excitation, respectively. The findings will form the basis for novel near-field engineering with inhomogeneous or spatially structured illumination.