Rebuilding a defected bone by activating the innate self-regeneration ability of bone requires a considerably long period of time. The purpose of the present invention is to provide a bone defect filling material that initiates a bone rebuilding activity as quickly as possible after implantation and thereafter remains in the defect to continue promoting bone formation activity until sufficient bone formation has been achieved for the rebuilding of the defect. The present invention provides a cotton-like bone defect filling material comprising biodegradable fibers produced by electrospinning. The biodegradable fibers contain 40-60 wt % of calcium phosphate particles and 10 wt % or more of silicon-releasing calcium carbonate particles, with the remainder containing 30 wt % or more of poly(L-lactic acid) polymer, and the amount of the poly(L-lactic acid) polymer that is non-crystalline is 75-98%.
