X-ray absorption near edge structure (XANES) has been applied to the elucidation of electronic structures in conduction-band region of SiO_x and SiN_x produced by ion implantation in Si (100). The XANES spectra of SiO_x at the Si2p edge are similar to those of SiO₂ but the spectral structure of intermidiate compound such as SiO is not observed. This fact suggests that the SiO_x layer consists of a mixture of Si and SiO₂ islands, and the conduction band of SiO_x is mainly composed of the orbitals of SiO₂. On the other hand, the XANES spectra at the Si2p edge of SiN_x gradually change depending on the x-values, and a sharp resonance corresponding to the N-dangling appears at the N1s edge for x>1.0. These observations suggest that SiN_x phase prefer randombonding structure rather than a mixture of Si and Si₃N₄ islands.