Composition is hardenable and comprises component(s) which is oxidizing compound or capable of releasing oxidizing compound in presence of water. Independent claims are included for the following: (1) kit, which comprises paste (A) and paste (B). The paste (A) contains radical polymerizable monomer(s) (a1), organic polymer(s) (a2) soluble in radical polymerizable monomer(s) (a1), and polymerization inhibitor(s) (a3). The paste (B) contains radical polymerizable monomer(s) (b1), organic polymer(s) (b2) soluble in radical polymerizable monomer(s) (b1), and polymerization accelerator(s) (b3). At least one of paste (A) and/or (B) contains hydrogen peroxide compound (s) as component (a4) or (b4)； (2) hardenable bone cement, which is obtained by mixing paste (A) and (B), or powder component (C) and monomer component (D)； and (3) molded article, which is obtained by polymerizing the composition or by mixing paste (A) and (B), or powder component (C) and monomer component (D) and polymerizing.
