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.