Si-added amorphous carbon (a-C) nanoparticles with particle diameter of 12.6 nm were synthesized using radio frequency (r.f.) plasma-enhanced chemical vapor deposition method equipped with porous aluminum plate in plasma. The optical gap of Si-added a-C particles with Si/(Si + C) ratio of 0.51 was 1.92 eV. Band edges of this nanoparticles was estimated to be as follows: i) conduction band edge was located over the standard potential of H+/H2 and ii) valence band edge was located under the standard potential of O2/H2O. From X-ray photoemission spectra, it was clarified that the part of tri-s-triazine structure with higher adsorption property toward water was introduced into nanoparticle with optical gap of 1.92 eV by adding graphitic-carbon nitride (g-C3N4) in a source material. The photo-oxidation measurement of indigo carmine adsorbed on Si-added a-C nanoparticles with tri-s-triazine structure was carried out. As a result, indigo carmine was decomposed at a rate of 0.0197 μmol/h. under UV irradiation. This result indicates that Si-added a-C nanoparticle can be applied to photocatalyst for reduction of water (estimated quantum efficiency of 0.049 %). It was experimentally demonstrated that the proposed Si-added a-C nanoparticles are tailor-made photocatalyst material that enables us to adjust band edge and surface functionalities for photo-oxidation (reduction) reactant.