We experienced a case of ventilation impairment resulted by the wrong-assembled unidirectional valves passed the automatic checkout of ventilator during general anesthesia. A 63-year-old man was scheduled for Ivor-Lewis operation due to esophageal cancer. After endotracheal intubation, ventilation impairment and abdominal distension were developed. No abnormal finding was found with bronchofiberoscopic examination. Suspecting anesthetic equipment failure, unidirectional valves were separated and they were proved to be connected upside-down.