A 68-year-old woman who underwent distal pancreatectomy combined with resection of the celiac axis had an abdominal wall recurrence and resection 2 years after the first surgery. She was treated with S-1 at an outpatient clinic following the surgery. She returned to the clinic with a high fever after the S-1 administration and was diagnosed with febrile neutropenia. However, treatment for febrile neutropenia including antibiotics and G-CSF did not improve her symptoms. Her history of chronic severe neck pain and painful enlargement of her thyroid gland suggested thyroiditis. After blood tests and thyroid scintigraphy, she was diagnosed with subacute thyroiditis and was treated with prednisolone. Her symptoms improved within a week. Although patients with neutropenia and a high fever during chemotherapy are likely to have febrile neutropenia, the possibility of another cause of neutropenia with fever should be considered if treatments for febrile neutropenia are not effective.