An 80-year-old Japanese woman with wet cough and dyspnea was diagnosed with pneumonia at a clinic. Antibiotics did not improve her symptoms; therefore, she was referred to our hospital one month after symptom onset. Chest radiograph findings revealed complete collapse of the left lung. Bronchoscopy showed white mucus plug in the left main bronchus, which could not be removed. She was initially treated with bromhexine. Subsequently, culture results of the mucus plug specimen obtained during bronchoscopy yielded Schizophyllum commune. After three weeks, improvement of the collapsed lung was observed on chest radiograph.