To make volume reduction molding under stable operation possible even if kind of waste plastic under continuous feeding is changed by a method wherein while temperature of a die and a barrel of a screw type extruder are measured, a heating fluid or a cooling fluid is circulated in a jacket on the outer peripheral face to indirectly heat or cool the inside of the machine.

A jacket 11 is arranged on the outer peripheral face of a casing 10 and inlet and outlet flanges 11a and 11b for introducing and discharging selectively hot water or steam and heating/cooling pipeline systems are provided. In addition, a recessed hole and a key channel are provided on the apex face of a screw 13 and a fixed shaft 31 is freely rotatably supported by a die 16A on the outer peripheral face of which a band heater 24 is clamped through a bush 32 and a die end plate 16B and a temp. detector 12 is provided on the top face of the outer periphery of the apex part of the die end plate 16B and a casing 10 to control the heating and cooling fluids thereby. In addition, a cutting unit 30 is constituted of the shaft 31, a cutting member 33 and a cutting blade 34 to cut a rod-like extrudate extruded from a nozzle opening 17a.