A 39-year old man presented with fever, cough, and dyspnea on exertion. High-resolution computed tomography showed diffuse ground-glass opacities and centrilobular nodules in bilateral lung fields. Video-assisted thoracic surgical biopsy specimens revealed noncaseating epithelioid granulomas with lymphocytic alveolitis. Hypersensitivity pneumonitis was suspected according to these results. He had worked as a baker for 10 years, and he had begun making bread with homemade baker's yeast just before the onset. Saccharomyces cerevisiae was isolated from cultures of the homemade yeast. Precipitating antibodies against the extract of the S. cerevisiae were present in his serum. An environmental provocation test using original homemade baker's yeast yielded a positive result. He was diagnosed as hypersensitivity pneumonitis induced by S. cerevisiae. After he cleaned up his bakery, started using a surgical mask, and stopped using homemade baker's yeast, he was able to restart making bread without recurrence. This is the first case of the hypersensitivity pneumonitis of a baker induced by S. cerevisiae.