A 41-year-old man was admitted to our hospital with a productive cough and dyspnea on exertion. His house and the bakery where he worked were made of wood, and both were built more than 40 years ago. Computed tomography of the chest showed diffuse ground-glass attenuation and centrilobular nodules in the bilateral lungs. Bronchoalveolar lavage fluid from the right S⁴ showed lymphocytosis and a low CD4/CD8 ratio. Specimens obtained by transbronchial lung biopsy from the right S⁸ revealed alveolitis in the interstitium and Masson bodies in the centrilobular regions. The titer of anti-Trichosporon asahii antibody was high. Based on these findings, the symptoms, and the clinical course, the patient was diagnosed as having summer-type hypersensitivity pneumonitis (SHP). After admission, his symptoms and the abnormal shadows in the bilateral lungs rapidly improved without therapy. To determine the offending environment for the SHP, we performed separate environmental provocation tests in his house and workplace. The provocation test result conducted in the workplace environment was positive; we therefore finally diagnosed SHP which was caused by the workplace. This important case demonstrated that a detailed interview and environmental provocation tests were essential for diagnosing SHP.