A 16-year-old female patient had influenza and was treated by inhalation of an anti-influenza agent. However, two days later, she showed slight dyspnea, and massive atelectasis of the left lung with hypoxia was found. She was admitted to our hospital (Day 0) and started on antibiotics and steroids, but the atelectasis was worsend on Day 3. Therefore, bronchoscopy was performed on Day 5. After the pus in the left bronchus was removed, the atelectasis was significantly improved. Atelectasis after influenza may be a rare, but critical complication, and bronchoscopy should be considered to release the obstructive lesions.