Guidelines advise precautionary measures for possible adverse events that may occur due to sedation during endoscopic procedures. To avoid complications, intraprocedural and postprocedural monitoring during recovery is considered important. However, since not many studies have reported on hypoxemia during the recovery period, findings for specific monitoring methods are insufficient. The aim of this retrospective study was to determine the incidence of hypoxemia during the recovery period using continuous central-monitoring by pulse oximetry and to characterize the hypoxemia cases. Among the 4065 consecutive esophagogastroduodenoscopy (EGD) procedures under planned moderate sedation, 84 (2.1%) procedures developed unexpected hypoxemia (SpO2 ≤ 90%). Hypoxemia was observed during the procedure, at the end of the procedure, and during the recovery period in 21, 17, and 46 (1.1%) procedures, respectively. More than half of the hypoxemia cases occurred during the recovery period. Many hypoxemia cases were characterized by neither serious co-morbid illness nor low body mass index which have been reported as risk factors of hypoxemia. The lack of risk factors is no guarantee that hypoxemia will not occur. Therefore, continuous monitoring by pulse oximetry is more important during the recovery period and is recommended in all EGD procedures under planned moderate sedation.