To provide a thermoplastic resin composition for injection molding affording a thin-wall injection-molded item that has high thermoconductivity, high insulating properties and high heat resistance, exhibits excellent flowability on injection molding and is suitable as a heat-dissipating member of an electronic/electric apparatus and the like.

The thermoplastic resin composition for injection molding comprises a thermoplastic resin component comprised of polymethylpentene (PMP) and polyphenylene ether (PPE), magnesium oxide, maleic anhydride-modified polypropylene and a glass fiber with the weight ratio of the polymethylpentene (PMP) to the polyphenylene ether (PPE) of 95:5 to 50:50.