A bioelectronic sniffer device for trimethylamine (TMA) in the gas phase "fish-odor substance" was constructed using a flavin-containing monooxygenase 3 (FMO3, one of xeno-biotic metabolizing enzymes) and a reaction unit with both gas and liquid cells separated by a porous poly(tetrafluoroethylene) diaphragm membrane (pore size: 30-60 μm, thickness: 0.20 mm). A substrate regeneration cycle was applied to the FMO3 immobilized device in order to amplify the output signal by coupling the monooxygenase with a reducing reagent system of ascorbic acid (ASA) in phosphate buffer. The sniffer device with 10.0 mmol/l AsA could be used to measure TMA vapor from 0.52 to 105 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). Since the application of the substrate regeneration cycle was possibly successful, it improved the sensitivity of the FMO3 immobilized device. The sniffer device possessed high selectivity for TMA being attributable to the FMO3 substrate specificity, continuous measurability, and good reproducibility in the repeatedly measurements (coefficient of variation = 2.41%, n=10).