An artificial meniscus, a base material of which is a hydrogel having a semi-interpenetrating network structure or an interpenetrating network structure (e.g., a linear polymer or a network structure composing the hydrogel is a polymer of an electrically charged unsaturated monomer and/or an electrically neutral unsaturated monomer or a crosslinked product thereof or/and a naturally occurring macromolecule or a crosslinked product thereof.), which can maintain moderate mechanical properties even in the case of application for an extended period of time between the joints where the severest mechanical environment is found in an organism, and which has no fear of causing osteoarthritis.