  We analyze the resonant inelastic x-ray scattering (RIXS) spectra at the Cu
and Ni K edges in La2CuO4 and La2NiO4, respectively. We make use of the
Keldysh-Green-function formalism, in which the RIXS intensity is described by a
product of incident-photon-dependent factor and density-density correlation
function in the 3d states. The former factor is calculated using the $4p$
density of states given by an ab initio band structure calculation and the
latter using the wavefunctions given by a Hartree-Fock calculation of a
multi-orbital tight-binding model. The initial state is described within the
Hartree-Fock approximation and the electron correlations on charge excitations
are treated within the random phase approximation. The calculated RIXS spectra
well reproduce several characteristic features in the experiments. Although
several groups have interpreted the RIXS peaks as bound excitons, our
calculation indicates that they should be interpreted as band-to-band
excitations augmented by electron correlations. The difference in RIXS spectra
between La2CuO4 and La2NiO4 is explained from this point of view.
