A 52-year-old woman visited our hospital with a chief complaint of a mass in the left breast. Palpation revealed a mass approximately 3 cm in diameter located on the outer aspect of the left breast. Breast ultrasound revealed a hypoechoic area with an indistinct border, which suggested breast cancer or mastitis. A needle biopsy revealed no evidence of malignancy, and granulomatous mastitis was diagnosed. Antibiotic therapy showed no improvement; therefore, we began prednisolone treatment for the patient. The mass diameter decreased over the subsequent 5 months after initiating this treatment, and we decreased the dose of prednisolone gradually. At present, when 8 months have elapsed after the diagnosis, the patient is doing well without any aggravation of signs or symptoms. The pathological findings of an inflammatory pseudotumor and granulomatous mastitis is similar, relatively rare benign diseases. As the inflammatory pseudotumor has clinical features resembling those of breast cancer, it is an important clinical entity.