A 46-year-old man visited our hospital with a fever and cough. The symptoms had started two months after continued use of an ultrasonic humidifier. He had hypoxemia on admission and late inspiratory crackles in both lungs on physical examination. The laboratory findings showed an increased white blood cell count and a C-reactive protein level, and his serum KL-6 level was slightly elevated, at 674 U/mL. Chest computed tomography showed diffuse ground-glass opacities, and histological examination of a transbronchial lung biopsy showed alveolitis without granulomas. The humidifier inhalation challenge test result was positive. Therefore, we diagnosed the patient with humidifier lung. His symptoms gradually improved after avoiding the humidifier without taking medication. The humidifier water was contaminated by various bacteria and fungi, as well as Mycobacterium gordonae and a high concentration of endotoxin. Unlike in those with typical hypersensitivity pneumonitis, the elevation of serum KL-6 levels in humidifier lung patients is mild, and granulomas are not apparent on histological examination, similar to our case. Furthermore, the endotoxin identified from the humidifier is one of the known pathogens of humidifier lung. Thus, humidifier lung seems to have different characteristics compared to other hypersensitivity pneumonitis phenotypes. The mechanism driven by the high concentration of endotoxin could be one of the main causes of humidifier lung.