A 27-year-old, previously healthy woman was admitted to our hospital for mild pneumonia. After 2 days ceftriaxone sodium administration, her chest radiograph revealed a rightward mediastinal shift caused by atelectasis of the upper portion of the right lung. Bronchoscopic examination showed swelling in the right upper lobe bronchus and obstruction in the B-1 segmental bronchus caused by complete edematous swelling. Histopathology showed acute cellular bronchitis with edema of the bronchial wall containing lymphocytes, plasma cells, and macrophages. Mycoplasma pneumoniae was detected by culture and a polymerase chain reaction test using sputum collected during bronchoscopy, and treatment was changed to minocycline. After 7 days antibiotic therapy, her condition improved and no relapse was observed. Identification of point mutations in domain V of the 23S rRNA for macrolide-resistant M. pneumoniae was performed, and an A-to-G transition at position 2063 in domain V of the 23S rRNA gene was identified. Atelectasis caused by M. pneumoniae is thought to be a common associated finding in pediatric patients, but it is rare in adults. In addition, our patient showed extremely unusual findings with obstruction caused by complete edematous swelling.