Background: Systemic chemotherapy with anti-HER2 therapy is the standard of care for HER2-positive advanced breast cancer. Patient outcomes have improved remarkably with the use of novel anti-HER2 drugs, including trastuzumab (H), pertuzumab (P), and trastuzumab emtansine (T-DM1). The combination treatment comprising H, P, and docetaxel (D) (HPD) is highly recommended as the 1st-line treatment for patients with HER2-positive advanced breast cancer. In contrast, for elderly patients over 65 years of age, this regimen seems to be intolerable mentally and physically, and impairs their quality of life. A new standard treatment with less toxicity and non-inferior efficacy for elderly patients is needed. Methods: We have planned a randomized, multicenter, open-label, phase III trial to confirm the non-inferiority of T-DM1 compared to HPD in terms of overall survival (OS) in elderly patients with HER2-positive advanced breast cancer. The eligibility criteria are as follows: 1) histologically proven metastatic breast cancer, 2) age 65-74 years with a performance status (PS) score 0-2, or 75-79 years with a PS score 0-1, 3) HER2 overexpression or amplification confirmed in primary or metastatic tissues, and 4) no anti-HER2 therapy with chemotherapy for breast cancer, excluding (neo) adjuvant therapy. Patients are randomized to receive either HPD (H 6 mg/kg, P 420 mg/body, and D 60 mg/m²) or T-DM1 3.6 mg/kg every 3 weeks. The dose up of D (75 mg/m²) after the second cycle is defined based on the data regarding safety during the first cycle. The primary endpoint is OS. The secondary endpoints are progression-free survival, response rate, adverse events, breast cancer-related death, and deterioration of activities of daily living. The trial is designed to achieve 70% power to confirm non-inferiority of T-DM1 to HPD at a one-sided alpha of 5% with a non-inferiority margin of 1.3 in terms of hazard ratio. With a median OS of 30 months in both arms, 6 years of accrual, and 5 years of follow up, the planned sample size is 330. The trial began in January 2018 and nineteen patients have been enrolled as of January 2019. Clinical trial information: UMIN000030783.