The case involved a 63-year-old female who had complained of a hard reddish lump in the right breast. Although the lump initially appeared to be a phyllodes tumor, histopathology after initial surgery indicated mastopathy as a likely diagnosis. Two years later, regrowth of the lump at the same location prompted revaluation. Ultrasound scan (US) revealed the following features: a) an irregular low-signal lesion with a diameter of 5 cm in the right A region extending toward the hypertrophic epidermis, b) marked increase in blood flow on power Doppler, c) increased thickness between the skin and underlying pectoralis major, and d) irregular and unclear margin of low-echoic lesion. Based on those findings along with clinical symptoms, advanced stage of inflammatory breast cancer was suspected, and a second surgery was performed. Histopathology revealed infiltration of atypical spindle cells around normal mammary ducts, but it did not show the architecture of a phyllodes tumor. The atypical spindle cells were positive for vimentin and CD34 and negative for hormone receptors according to immunohistochemistry. These were consistent with the characteristics of periductal stromal sarcoma. The section diagnosed as mastopathy 2 years previously was re-evaluated by pathologists, and components of stromal sarcoma were found in the section. Stromal sarcoma is rare for the breast region and usually lacks specific features, which may make the precise diagnosis quite difficult. In our case, the following factors contributed to the misleading diagnosis: invasive growth without forming a firm mass, less prominent nuclear pleomorphism and low proliferation index, and inconsistent US features compared to the previous report, etc. The generally poor prognosis of this tumor should demand long-term observation even though no recurrence has been found after the second surgery.