A kneading apparatus of a thermoplastic resin, a plasticizing cylinder, a high pressure mixing zone in which the thermoplastic resin is in contact kneading and the pressurized fluid molten resin which has been melted plasticized, the pressurized fluid is contacted kneading the decompression zone and plasticizing cylinder, a screw which is rotated and retractably disposed in said plasticizing cylinder, and the high pressure mixing zone and a decompression zone for exhausting the pressurized fluid which is gasified from the molten resin that is provided between the downstream sealing mechanism for shutting off the communication between the high pressure mixing zone and the decompression zone, to connect to the decompression zone of the plasticizing cylinder, the downstream sealing mechanism and the high pressure mixing zone when you cut off the communication between the pressure reducing zone, controlling the pressure of the reduced pressure zone under the maximum pressure of the high-pressure kneading zone to reach when and contacting kneading the pressurized fluid to the molten resin at atmospheric pressure or higher to provide a kneading apparatus having a reduced pressure zone pressure adjustment mechanism.