The present invention provides: a process for producing a high molecular weight thermoplastic resin composition, which can give the high molecular weight thermoplastic resin composition with good reproducibility and stably; and uses of the resultant resin composition. The process for producing a high molecular weight thermoplastic resin composition comprises the step of mixing a thermoplastic resin with pyromellitic dianhydride to carry out a reaction therebetween to convert the thermoplastic resin into a high molecular weight one, and this process is characterized in that the pyromellitic dianhydride has at least one of the following properties in particle form: (1) intrinsically linear or needle-like particle forms; (2) a bulk density of 0.4˜0.9 g/ml; and (3) a rest angle of 50˜70 degrees.