Hypersensitivity pneumonitis due to the inhalation of oyster mushroom(pleurotos ostreatus) was demonstrated in a 44-year-old man who had cultiviated for 4 years. He had suffered from dyspnea, cough, myalgia and fever in the evening after working at first. After them he has experienced dyspnea aggravated. He showed not only positive reaction to the intradermal test but also to the bronchial challenge test such as 6 hr after inhalation falling down of FVC, FEV p, and DLCO in pulmonary function test and leukocytosis with fever upto 38.2C with antigenic extract of oyster mushroom spore. The size of spore was approximately 3.5 when measured by transmission electron microscopy.