During laparoscopic surgery, carbon dioxide (CO₂) pneumothorax can develop due to a congenital defect in the diaphragm. We present a case of a spontaneous massive left-sided pneumothorax that occurred during laparoscopy-assisted gastrectomy, because of an escape of intraperitoneal CO₂ gas, under pressure, into the pleural cavity through a congenital defect in the esophageal hiatus of the left diaphragm. This was confirmed on intraoperative chest radiography and laparoscopic inspection. This CO₂ pneumothorax caused tolerable hemodynamic and respiratory consequences, and was rapidly reversible after release of the pneumoperitoneum. Thus, a conservative approach was adopted, and the remainder of the surgery was completed, laparoscopically. Due to the high solubility of CO₂ gas and the extra-pulmonary mechanism, CO₂ pneumothorax with otherwise hemodynamically stable conditions can be managed by conservative modalities, avoiding unnecessary chest tube insertion or conversion to an open procedure.