The present invention relates to a novel reactive medical substance having the bone restoration function, comprising a particular methacrylic acid ester, polymer and osteogenic factor. The reactive medical substance of the present invention has a mechanical strength which is increased by a reaction and is used in a living body for a long period of time. The osteoinduction-assistance composition having the bone restoration function of the present invention is characterized in that the composition maintains suitable elasticity, and can be easily formed with scissors or a sharp blade upon use. The reactive medical substance of the present invention comprises a mixture of a methacrylic acid ester, a polymer and a biological absorbing material, and can regenerate a mocosa, form a regenerated epithelium, and stabilize a transplanted piece.