Intubation of an double-lumen endotracheal tube (DLT) in either a normal or a difficult airway may be more difficult than the intubation of a single-lumen endotracheal tube (ETT). A 72-year-old man undergoing right upper lobectomy could not be intubated with either a 37 Fr or a 35 Fr DLT after induction of anesthesia due to an inability to visualize the glottis. Difficult intubation had not been foreseen, but inspection through the laryngoscope revealed a Cormack grade III condition. Following two attempts to place the DLT, fibroptic bronchoscopy also proved unsuccessful. We intubated a 8.0 mm ETT with OELM (optimal external laryngeal manipulation) and followed this by inserting an infusion set catheter into the ETT as a guiding stylet. The ETT was then withdrawn with the catheter in situ in the trachea, and a 35 Fr DLT successfully introduced. After removing the catheter, the position of the DLT was corrected by fibroptic bronchoscopy. No further complication occurred and surgery was finished satisfactory. We achieved successful DLT intubation using the modified Seldinger technique with an infusion set catheter through an intubated ETT in a patient with an unanticipated difficult airway.