The popularity of a sling method using biomaterial sheets for immediate breast reconstruction based on prosthesis has been increasing in western countries. However, acellular dermal matrix, which is representative of the biomaterial sheet, is not available and the sling method also has not been accepted in Japan. We focused on a new bioabsorbable sheet (NEOVEIL sheet) as a substitute for the sling method and report a prospective study protocol to assess the safety and effectiveness of this material. This was an ongoing, single center, open-label, single-arm study. Inclusion criteria and exclusion criteria are defined restrictively. If the surgeon determined that the perfusion of skin envelope after mastectomy is poor, the surgical procedure can be modified and that patient was excluded from the study. The primary outcome was the incidence of tissue expansion or implant explantation occurring within 1 month after surgery. The secondary outcomes are as follows: (1) aesthetic outcome using a rating scale; (2) symmetry of the nipple areolar complex position; (3) patient reported outcomes using BREAST-Q; (4) pain intensity using the Visual Analog Scale; (5) histology of the capsule around the tissue expander; (6) inflation volume at the first stage and overall inflation time of expansion; and (7) other adverse events regarding the surgery. This study will determine the safety and effectiveness of the sling method using a NEOVEIL sheet in Japanese women.