The Breast Cancer Group of the Japanese Radiation Oncology Study Group conducted a nationwide questionnaire survey on the clinical practice of postoperative radiotherapy for breast-conserving treatment for breast cancer. This questionnaire consisted of 18 questions pertaining to the annual number of treated patients, planning method, contouring structure, field design, dose-fractionated regimen, application of hypofractionated radiotherapy, boost irradiation, radiotherapy for synchronously bilateral breast cancer, and accelerated partial breast irradiation. The web-based questionnaire had responses from 293 Japanese hospitals. The results indicated the following: treatment planning is performed using relatively similar field designs and delivery methods; the field-in-field technique is used at more than one-third of institutes; the commonest criteria for boost irradiation is based on the surgical margin width (≤5 mm) and the second commonest criteria was age (≤40 or ≤50 years), although some facilities applied a different age criterion (>70 years) for omitting a tumor bed boost; for conventional fractionation, almost all institutes delivered 50 Gy in 25 fractions to the conserved whole breast and 10 Gy in 5 fractions to the tumor bed. This survey revealed that 43% of hospitals offered hypofractionated radiotherapy, and the most common regimens were 42.56 Gy in 16 fractions for whole-breast irradiation and 10.64 Gy in 4 fractions for boost irradiation. Almost all of the facilities irradiated both breasts simultaneously for synchronously bilateral breast cancer, and accelerated partial breast irradiation was rarely offered in Japan. This survey provided an overview of the current clinical practice of radiotherapy for breast-conserving treatment of breast cancer in Japan.