  We study how Co- and Ni-substitution affect the electronic structure of the
iron-based superconductor, LaFeAsO. We perform {\it ab initio} supercell
calculations and unfold the first Brillouin zone (BZ) to calculate the spectral
function in the BZ for the normal cell. The charge density distribution in real
space shows that doped extra electrons are trapped around Co (Ni) atom. This
seems to mean that Co(Ni)-substitution does not work as carrier doping.
However, the present momentum-space analysis indicates that the Fermi-surface
volume indeed expands by substitutions, which can be well described by the
rigid-band shift. By taking into account this effective doping, we discuss
whether the sign-reversing s-wave ($s_{\pm}$-wave) scenario is compatible with
experiments.
