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A bio-optical gas-sensor (sniffer device) was constructed by immobilizing flavin-containing monooxygenase 3 (FMO3, one of xenobiotic metabolizing enzymes for catalyzing the oxidation of odorous substances such as trimethylamine: TMA) onto a tip of a fiber optic oxygen sensor with oxygen sensitive ruthenium organic complex (excitation: 470 nm, fluorescent: 600 nm) with a tube-ring. A reaction unit for circulating buffer solution was applied to the tip of the sniffer device. A substrate regeneration cycle was applied to the FMO3 immobilized sensor in order to amplify the output signal by coupling the monooxygenase with a reducing reagent system of ascorbic acid (AsA) in phosphate buffer. The bio-optical sniffer was possible to detect the oxygen consumption induced by FOM3 enzymatic reaction with TMA application. The sniffer device with 10.0 mmol/1 AsA could be used to measure TMA vapor from 0.31 to 125 ppm, this covers the maximum permissible concentration in the work place (5.0 ppm of Time Weighted Average concentration) and the sensing level-5 of smell in humans (3.0 ppm). The sniffer device possessed high selectivity for TMA being attributable to the FMO3 substrate specificity, continuous measurability.