We investigated the chemical states of the interfaces between hydrofluoric acid solution and a silicon (Si) substrate using infrared absorption spectroscopy in multiple internal reflection geometry (MIR-IRAS). At the initial stage of porous Si formation, Si atoms on the outermost layer were removed and those on the second layer were terminated with hydrogen atoms to generate monohydride species bonded to hydride species (SiH^*). The data of time evolution of the hydride species density obtained by etching simulation using the Monte Carlo method agreed fairly well with the experimental data. And it was found that the etching rate of SiH^* is comparable to that of dihydride species (SiH₂). At the high anodic current density region, where anodic current density is constant but anodic potential oscillates, we observed generation of suboxide Si(O₃)-H at a minimum anodic potential. However, the suboxide was dissolved in the solution immediately after an increase in the anodic potential. This indicates that the formation and dissolution of an inhomogeneous oxide layer play an important role for the galvanostatic oscillation.