To provide a process for producing thermoplastic resins which reduces unreacted components and byproducts in a tandem-type reaction extruder.

A tandem-type reaction extruder includes the first extruder in which a sold resin material is plasticized, auxiliary raw materials for the first step reaction are supplied for reaction, the second extruder in which auxiliary raw materials for the second step reaction are fed into the molten resin supplied from the first extruder and reaction and/or deaeration is conducted, a part connecting the outlet for the resin of the first extruder with the raw material inlet of the second extruder, and a pressure control mechanism for the inside of the first reactor. A control mechanism for the pressure inside of the part connecting the outlet for the resin of the first extruder with the raw material inlet of the second extruder is additionally arranged at the raw material inlet of the second extruder of the tandem-type reaction extruder. The pressure difference P between the pressure on the side of the first extruder and the pressure on the side of the second extruder is controlled at 2 MPa and 10 MPa.