A 65-year-old woman suffered from swollen parotid and submandibular glands in July 2005, and sicca of the eyes and oral cavity arose in October 2005. We diagnosed her as having Mikulicz's disease based on magnetic resonance imaging of the head, gallium scintigraphy, and a lip biopsy in June 2006. After prednisolone 10 mg/day was commenced, her salivary glands shrank slightly. Although the amount of prednisolone had been tapered to 7 mg/day by May 2007, her serum IgG rebounded. In January 2008, hachimijiogan extract granules 7.5 g/day were introduced. Swelling of the salivary glands disappeared and her serum IgG level decreased. In May 2008, the amount of prednisolone was tapered successfully to 6 mg/day. Mikulicz's disease is a corticosteroid-sensitive disease, but in this case steroid resistance made treatment difficult. Based on this case, we performed functional assays of P-glycoprotein with calcein-AM, which demonstrated that hachimijiogan can reverse drug resistance.