It is common to use semiclosed circle system for general anesthesia because of economy, easy controllability of humidity and temperature, less contamination of operating room with anesthetic gas etc, but in case of malfunction of unidirectional valve, inappropriate absorption of carbon dioxide,leakage or obstruction of circle system the patient may become fatal. During general anesthesia with newly purchased anesthesia machine(Ohmeda Modulus II plus) we experienced three cases of hypercarbia because of malfunction of expiratory valve which could not be easily found due to overlying PEEP valve.