A 69-year-old woman presented with obstructive jaundice and a 30-mm hypoechoic mass in the pancreatic head on ultrasonography. Magnetic resonance imaging (MRI) revealed enlargement of the pancreatic head with dilatation of the upstream main pancreatic duct and no dilatation of the proximal biliary tree. Endoscopic retrograde pancreatography showed a localized irregular narrowing of the main pancreatic duct in the head of the pancreas. Pylorus-preserving pancreatoduodenectomy (PPPD) was performed under the diagnosis of pancreatic head cancer. Histopathological examination showed fibrosis with lymphoplasmacytic infiltration, suggesting the diagnosis of autoimmune pancreatitis (AIP). Serum IgG concentration was within normal limits immediately after the operation but was elevated 4 months later, when MRI showed enlargement of the remnant pancreas, with a peripheral rim of low intensity. Oral administration of prednisolone was initiated, at a dose of 5 mg/day. The serum IgG concentration declined and MRI showed improvement of the pancreatic enlargement and the disappearance of the peripheral rim. AIP has not relapsed for 1 year so far, with the patient being kept on 5 mg/day prednisolone. This communication reports a patient with AIP showing an interesting clinical course.