Optical polarization anisotropy on the surface of subwavelength air-dielectric structures was studied by polarization-modulation near-field optical microscopy. A large degree of polarization anisotropy was measured in the higher topographic region of transparent gratings. In two-dimensional air-hole arrays, the largerpolarization anisotropy occurs in the dielectric regions. Both dielectric and air regions have a 90 degrees difference in the directions of maximum transmission. From calculations with the finite-difference time-domain method, we verified that diffracted light from the mesoscale topographic edges contributes to polarization anisotropy and directional differences.