PURPOSE: To obtain the subject catalyst capable of inhibiting the start of polymerizing reaction in a state before injection into a forming mold, but rapidly advancing the reaction after the injection into the forming mold by bringing specific two kinds of solely inactive compounds into contact according to a specified method and producing the catalyst.

CONSTITUTION: This polymerization catalyst expressed by the formula (R1)4N-+N(R2)4 is produced by respectively mixing (A) a compound expressed by the formula (R1)4B-+M (R1 is alkyl or aryl; M is alkali metal) and (B) a compound expressed by the formula (R2)4N+-(R3) (R2 is R1; R3 is halogen or OH) which are solely inactive to a thermoplastic resin raw material and do not form a highly active anion polymerization catalyst until both are mixed with the thermoplastic resin raw material, placing the resultant mixtures in separate containers, then mixing both and injecting the prepared mixture into a mold. Furthermore, e.g. sodium tetraphenylboride is cited as the component (A) and, e.g. tetrabutylammonium chloride is used as the component (B).