  We have studied the anisotropy in the in-plane resistivity and the electronic
structure of isovalent Ru-substituted BaFe$_2$As$_2$ in the
antiferromagnetic-orthorhombic phase using well-annealed crystals. The
anisotropy in the residual resistivity component increases in proportional to
the Ru dopant concentration, as in the case of Co-doped compounds. On the other
hand, both the residual resistivity and the resistivity anisotropy induced by
isovalent Ru substitution is found to be one order of magnitude smaller than
those induced by heterovalent Co substitution. Combined with angle-resolved
photoemission spectroscopy results, which show almost the same anisotropic band
structure both for the parent and Ru-substituted compounds, we confirm the
scenario that the anisotropy in the residual resistivity arises from
anisotropic impurity scattering in the magneto-structurally ordered phase
rather than directly from the anisotropic band structure of that phase.
