A fiber-optic biochemical gas sensor (biosniffer) for the continuous monitoring of acetaldehyde (AA) vapor was constructed and tested. The biosniffer was constructed using a flow cell with an aldehyde dehydrogenase (ALDH) membrane onto a fiber-optic reduced nicotinamide adenine dinucleotide (NADH) measurement system. AA vapor was measured as fluorescence of NADH produced by the enzymatic reaction of ALDH. A phosphate buffer that contained oxidized nicotinamide adenine dinucleotide (NAD+) was circulated into the flow cell to rinse NADH and the excessive substrates from the optode. A 340 nm ultraviolet light-emitting diode (UV-LED) was employed as an excitation source. The fluorescence of NADH (λ = 491 nm) was measured coaxially with a spectrometer. Characterization of the biosniffer was carried out using a standard AA measurement system. As a result, the typical response is between 0.1 to 5.0 ppm AA, which included the 8-h reference exposure level (0.16 ppm). The biosniffer is expected to be useful in the measurement of environmental AA.