We report a case of interval breast cancer that was an invasive ductal carcinoma of the breast with neuroendocrine differentiation. The patient was a 66-year-old woman who had a breast lump. She had received breast cancer screening every other year. The last screening was 1 year and 6 months prior to diagnosis. A core needle biopsy for breast tumor led to a diagnosis of invasive ductal carcinoma. Breast conserving surgery plus sentinel lymph node biopsy was performed. The pathological diagnosis from the resected surgical specimen was invasive ductal carcinoma with neuroendocrine differentiation, positive for ER and PgR, and negative for HER2/neu protein expression. The surgical margins were negative, and there was no metastasis in the sentinel lymph node. She was administered radiation therapy and adjuvant endocrine therapy. Two years after surgery, she is well without metastasis.