Breast-conserving therapy, where radiotherapy is performed after partial mastectomy, is a widely used surgical method that can preserve most of the breast tissue without increasing the recurrence rate of breast cancer. However, without reconstruction, asymmetry of the breast occurs due to the tissue defect and radiation fibrosis, producing in poor cosmetic results. In this case study, we performed staged prosthetic breast reconstruction combined with fat grafting for severe depressive deformation of the breast after breast-conserving therapy. The first surgery involved insertion of a tissue expander and fat grafting, second surgery was the reduction of injected saline volume and fat grafting, and third surgery involved exchange for an implant and fat grafting. The skin in the depressed area, which had atrophied, became soft and flexible; deformation was also improved, and the patient was satisfied with the aesthetic outcome. It is expected that fat grafting will fertilize and qualitatively improve the damaged tissue due to irradiation, reducing the complications related to the tissue expander and implant. We believe that it will be possible to correct a breast deformity after breast-conserving therapy using a tissue expander and implant, which had not been considered as a solution. The results showed that the present method can be an option for delayed reconstruction after breast-conserving therapy.