Abstract: An RIM method of thermoplastic resin comprises a process for preparing materials in which one compound (i) is mixed with a material for thermoplastic resin to prepare one mixture (I), and the other compound (ii) is mixed with a material for thermoplastic resin to prepare the other mixture (II), each of two compounds (i) and (ii) being inactive to the material for thermoplastic resin but producing a high-active anionic polymerization catalyst by their reaction, a process for polymerization in which the mixture (I) and the mixture (II) are mixed, injected into a mold, and polymerized by the anionic polymerization catalyst, an ejecting process in which a solidified thermoplastic resin molded body is ejected from the mold. Therefore, a polymerization reaction cannot be started before the material for thermoplastic resin is injected into the mold, and it begins soon after the material for thermoplastic resin is injected into the mold.