Unidirectional valve (UDV) malfunction causes rebreathing of expired gas during anesthesia. However, the resultant hypercarbia without hypoxemia by UDV malfunction is not easily detected. We experienced a case of severe hypercarbia which caused by sticking expiratory valve at 45 minutes after the induction of anesthesia, in spite of checking UDV function preoperatively. In this case, expiratory UDV malfunction was diagnosed with the change of capnographic waveform. And so, we recommend monitoring of capnographic waveform during every anesthesia, in addition to preoperative UDV checking.