A 58-year-old woman was admitted complaining of dry cough and exertional dyspnea. Physical findings, chest X-ray films, chest CT scan and respiratory function tests were suggestive of interstitial pneumonia. Transbronchial lung biopsy showed specific findings of hypersensitivity pneumonitis. As a result of positive provocation test using her home humidifier, a diagnosis of humidifier lung was made. Many microorganisms including Flavobacterium meningosepticum were cultured from the water left in the humidifier for one week. As both complement fixation test and precipitation test were positive to humidifier water and to extract of Flavobacterium meningosepticum, the humidifier and Flavobacterium meningosepticum were suggested to be causative in this case.