In designing bone fillers, hardness has heretofore not been a major concern. Fillers are typically very hard and thus often accelerate the collapse of the adjacent bones. We developed a novel, relatively soft bone filler, whose hardness is similar to the human cancellous bone. The structure is simple: a 0.14-mm-diameter pure titanium wire was rolled and folded with both ends buried in the central portion, resulting in a ball of 4-mm diameter with 83% internal void ratio, having 300-500 μm internal gaps. The balls are chemically washed in an acidic solution at the end of the manufacturing process. We call this new filling device titanium wire balls (TWBs). We implanted TWBs into the medial condyle of the right tibiae of twelve adult Japanese white rabbits, and histologically evaluated the results. Four weeks after implantation, the spaces in the TWBs were fully calcified; the TWBs, the calcified tissues in them and the cancellous bones surrounding them were all connected with each other. In conclusion, we developed a novel bone filler that has similar hardness to the human cancellous bone and an 83% internal void ratio, with 300-500 μm internal gaps. Four weeks after implantation, the spaces in TWBs were fully calcified and connected to the surrounding cancellous bones.