In this work, we assessed the ability of a fiber-optic biochemical gas sensor (bio-sniffer) to directly and nondestructively measure gaseous formaldehyde (FA) released from food. In the biosniffer, formaldehyde dehydrogenase (FALDH) was immobilized in a hydrophilic fluoropolymer membrane in which an enzymatic reaction producing nicotinamide adenine dinucleotide (NADH) occurred. The resultant NADH was excited with ultraviolet (UV) light irradiated from the optical fiber end in the bio-sniffer and the emitted fluorescence was coupled back to the fiber end and detected. In experiments to characterize the bio-sniffer using standard gaseous FA samples, the capability of continuous measurement (a coefficient of variation of 3.4% for five successive measurements) and high sensitivity (a limit of detection of 1.1 ppb) were validated. Then a demonstration of direct gaseous FA from food was carried out using Shiitake mushrooms (Lentinula edodes). Owing to the use of the enzyme, the bio-sniffer was able to selectively measure FA, and clear differences in FA concentrations were observed depending on the state of the mushrooms, i.e., dried, rehydrated with deionized water or with γ-GTPase inhibitor. These results indicated that the bio-sniffer allowed the direct monitoring of FA naturally produced in food or the evaluation of the metabolism of living organisms.