Although it is important to determine the risk factors for postoperative delirium (POD) to prevent the onset of POD, most previous studies that focused on risk factors for POD analyzed small cohort groups. Therefore, we aimed to explore the risk factors for the onset of POD by analyzing the National Health Insurance Claims Database (NDB) in Japan, which covers most Japanese residents.We compiled the clinical data of patients who emerged from POD after eight types of surgical procedures from August 2015 to August 2016 from the NDB. We subsequently developed a logistic regression model to determine the factors that affected the onset of POD.From the logistic regression analysis, we found that four factors (being over 75 years old, previous history of delirium, administration of a blood product during the operation, and emergency hospitalization) increased the odds ratio of POD. These factors may decrease physiological function, thereby leading to the onset of POD. We also clarified that the odds ratio of POD in patients administered inhalation anesthetic alone was lower than in patients administered combined inhalation and intravenous anesthetics for coronary artery bypass grafting. This finding was in line with the results of a previous small cohort study.These findings imply that, to prevent the onset of POD, appropriate precaution for and appropriate selection of anesthetic agents is necessary for patients with the four POD risk factors.