We have developed a Scanning Near-Field Optic Microscope (SNOM), which is based on a commercial tuning-fork sensor, capable of measuring the topography and the optical signal, at the same time. We have measured the SNOM transmission, as well as the topography, of an RbTiOPO4 single crystal, which exhibits two kinds of macroscopic ferroelectric domains. A chemical selective attack has been performed to distinguish them. As a consequence, we obtained zones with a noticeable roughness (C-) in comparison with the flat aspect of the other ones (C+). The SNOM topography images have been compared to Atomic Force Microscopy ones, which has a better resolution. The changes observed in the transmission measurements are due to different effects: i) variations of the dielectric constant at the interface walls between domains; ii) light scattering, wavelength dependent, at the grains in the C-face. Finally, we have determined the angles formed at the domain-walls between inverted domains, which are measured to be higher than 90° and close to 135°, as could be expected, because of the combination between [110], [-110], [100] and [010] directions of the orthorhombic lattice.