Injection molding of styrene resin

A method of injection molding a styrene resin or a composition thereof, which comprises injection molding a molten molding resin material of a styrene-conjugated diene block copolymer containing 50% to 90% by weight of a styrene polymer or a molten molding resin material of a resin composition comprising 50% by weight or more of said styrene-conjugate diene block copolymer and 50% by weight or less of a styrene resin other than said styrene-conjugated diene block copolymer, wherein the injection molding is carried out at a barrel temperature of 180.degree. to 250.degree. C., and wherein said molten resin is injected and filled into a mold with a wall surface temperature of 110.degree. C. to 180.degree. C. at core and cavity portions within the mold, and the resin is cooled at a speed of not lower then 0.3.degree. C./sec. after being filled into the mold, and the molded product is demolded when the wall surface temperature of the core and cavity portions is lowered to the glass transition temperature or lower of the styrene polymer moiety in said block copolymer or a temperature of not higher then 90.degree. C.