It is intended to provide a medical material which enables a new method for cartilage tissue regeneration treatment based on an entirely new concept unlike a treatment method by transplantation of autologous cartilage tissue, a cartilage alternative or undifferentiated cells. The invention provides a bone filler for cartilage tissue regeneration comprising hydrogel having an interpenetrating network structure formed by two or more crosslinked network polymers or a semi-interpenetrating network structure formed by a crosslinked network polymer and a linear polymer. By filling the bone filler of the invention in a hole or a groove provided in subchondral bone just under damaged cartilage tissue, regeneration of the cartilage tissue or both the cartilage tissue and the subchondral bone can be promoted.