To provide a method for producing a resin composition having excellent heat resistance and moldability while making the best use of excellent impact resistance and mechanical strength of a polynorbornene-based molding product and to obtain the resin composition produced by the production method.

This method for producing the resin composition composed of a thermoplastic resin and a norbornene-based polymer comprises compounding 100 pts.wt. of the thermoplastic resin with 1-300 pts.wt. of a norbornene-based monomer and/or a norbornene-based oligomer, dispersing the norbornene-based monomer and/or the norbornene-based oligomer into the thermoplastic resin and polymerizing the norbornene-based monomer and/or the norbornene-based oligomer by using a metathesis polymerization catalyst.