Purposes: To clarify the incidence and risk factors of postoperative delirium in patients following pancreatic surgery, and the impact of yokukansan (TJ-54) administered to reduce delirium. Methods: Fifty-nine consecutive patients who underwent pancreatic surgery (2012.4-2013.5) were divided into 2 groups: TJ-54 group: patients who received TJ-54 (n = 21) due to insomnia and the No-TJ-54 group: patients who did not receive TJ-54 (n = 38), and the medical records including the delirium rating scale - Japanese version (DRS-J) were retrospectively reviewed. Results: Postoperative delirium occurred in 2 patients (9.5%) in the TJ-54 group and in 4 (10.5%) patients in the No-TJ-54 group (p = 0.90). The DRS-J on 5 days after surgery was lower in the TJ-54 group than in the No-TJ-54 group (rough p = 0.006), however, without any statistically significant differences with the Bonferroni correction. As for the hospital cost, there was no difference between the TJ-54 and the No-TJ-54 groups (p = 0.78). History of delirium was identified as an independent risk factor of postoperative delirium. Conclusion: The patients with preoperative insomnia, who were treated with TJ-54, did not have a higher incidence of postoperative delirium, compared to those without preoperative insomnia. The patients who had a history of delirium have an increased risk of postoperative delirium and should be cared for and treated prophylactically to prevent it.