To obtain a flame-retardant methacrylic resin compsn. capable of preventing corrosion of a metal casting or molding mold by formulating (A) A a methyl methacrylate resin with (B) a halogen-contg. phosphoric ester compd. and (C) a tin compd.

(A) 100 pts.wt. methyl methacrylate resin is formulated, with (B) 20-40 pts.wt. halogen-contg. phosphoric ester compd. and (C) 0.01-1.0 pt.wt. tin compd. An example of the component (A) is a polymer of methyl methacryalte, which can be lowered in the combustion rate of the resin when it is used in combination with a polyfunctional allyl compd. having 2 allyl groups as a monomer copolymerizable therewith. Examples of the component (B) include halogen-contg. polyphosphates. An example of the component (C) is di-n-octyltin-S,S'-bis(isooctylmercapto) acetate. For example, 2,2'-azobis(2,4- dimethylvaleronitrile) is used as a polymn. initiator, the amt. of which is 0.001 to 1.0 pt.wt. per 100 pts.wt. polymerizable starting materials.