Being a second order optical phenomenon, resonant inelastic X-ray scattering (RIXS) spectroscopy can be used to disentangle bound-state excitations from continuous absorption spectrum more effectively than ordinary first order absorption spectroscopy. When the excitation energy is lower than the K absorption threshold by about 10 eV or more, RIXS spectra observed approach a mirror image of 1s-core-hole lifetime-broadening-removed (LBR) XANES spectra, although the scattering intensity is prohibitively weak because of the unsatisfactory resonant condition employed. Approaching closer to the resonance, RIXS intensity increases by several orders of magnitude and concurrently observed is a surprisingly complicated set of spectra which heavily depends on the excitation energies. From each RIXS spectrum obtained under close to the resonant conditions, 1s-core-hole LBR-XANES or 1s- and 2p-core-hole LBR-XANES can be deduced by analytical method or numerical calculations. The RIXS-XANES method has been applied to CuO to reveal an existence of hidden electronic states near the absorption threshold.