The occurrence of a pneumothorax during laparoscopy-assisted distal gastrectomy (LADG) is rare. A pneumothorax was developed during a LADG under general anesthesia in a 67-year-old woman with gastric cancer. About 140 minutes after CO₂ insufflation, sudden hemodynamic collapse occurred. A defect was noted in the diaphragm. After immediate repair under laparoscopy, hemodynamic stability was achieved within several minutes. In the anesthetic management of a LADG, the anesthesia provider should be aware of the possible occurrence of a pneumothorax.