We have been developing an on-site water monitoring system available for continuous monitoring of residual chlorine in environmental water. It is required that this system has high robustness to realize stable monitoring under severe environment during years. However, mineral precipitates and biofilm are formed in a microchannel and deteriorate absorbance measurement. In previous studies, we have proposed and fabricated a Diamond like carbon (DLC) coated microchannel. DLC coating can prevent mineral precipitate, but cannot prevent biofilm forming(1). Then we proposed to overlay the biocompatible 2-Methacryloxyethylphosphorylcholine (MPC) based copolymer on the DLC coating(2). (This is abbreviated MPC/DLC/glass) Mineral precipitates on the MPC/DLC/glass were more suppressed than those on the glass. In the case of Staphylococcus aureus(S. aureus), which is known to form biofilm, the bacteria adhesion on the MPC/DLC/glass was 20 % or more lower than that on the glass.