We experienced cases in which restoration volunteers at an enoki mushroom (Flammulina velutipes) cultivation farm damaged by the 2004 Mid Niigata Prefecture Earthquake developed influenza-like fever after work and visited our medical institution one after another. The questionnaire survey conducted in 21 of the 22 volunteers revealed that symptoms, such as fever (85.7%), general malaise (71.4%), and cough (76.2%), had occurred a mean of 8.7 ± 5.9 h after work in 20 volunteers. The environmental investigation revealed that a large amount of fungi had abnormally been proliferating and floating in the air at the workplace. They were identified as Aspergillus fumigatus. The same fungus was isolated from the specimens obtained from the respiratory tract in 2 patients who visited the medical institution. All patients spontaneously recovered without receiving antifungal drugs or steroids. Based on the above results, our cases were considered to be an epidemic outbreak of organic dust toxic syndrome (ODTS) caused by the inhalation of a large amount of organic dusts containing fungi. The levels of β-D-glucan were measured in 10 ODTS patients, of whom 6 had levels equal to or above the cutoff value. This result confirmed, for the first time, the potential use of β-D-glucan as a diagnostic aid in ODTS patients.