In the case of thickening a feed resin without pre-drying, a device is taught to enhance the thickening effect. The device includes a chamber 4 having a mixing rotor therein for continuously mixing a resin between a feed port 2 and a discharge port 3. A plurality of vent ports 7 connected to vacuum pumps 8, 9 are provided for reducing the pressure in the chamber. An element, e.g. a screw feeder 10, is provided for introducing a viscosity-increasing chemical agent into the chamber. The screw feeder is isolated from the reduced pressure areas of the chamber by elements 35, e.g. reverse screw flights of the rotor. In operation, feed resin from a hopper 6 enters the chamber through port 2 and is dehydrated while being continuously mixed in a drying section 14 via vents 7. After addition of the chemical agent, the resin is continuously mixed in a thickening section 15 while being deaerated.