An optical biosniffer for dimethyl sulfide (DMS; seaweed-odor substance) was constructed by immobilizing flavin-containing monooxygenase type 3 (FMO3) onto the tip of a fiber optic oxygen sensor with an oxygen-sensitive ruthenium organic complex (excitation: 470 nm, fluorescence: 600 nm) with a tube ring. A reaction unit for circulating buffer solution was applied to the tip of the sensor device. After the experiment in the liquid phase, the sniffer device was applied to gas analysis using a gas flow measurement system with a gas generator. The optical device could be applied to detect the oxygen consumption induced by the FMO3 enzymatic reaction with DMS application. The sensor in the liquid phase was used to measure DMS solution from 0.52 to 7.32 mmol/l, with a cyclic reaction with ascorbic acid (AsA) as the reducing reagent. Then, the sniffer device with the reaction unit was calibrated against DMS vapor from 2.1–126 ppm with good gas selectivity based on the FMO3 substrate specificity. The device could be used to monitor the concentration change of gaseous DMS using the reaction unit, in which the substrate and product substances flush out at the sensor tip and the enzyme membrane.