A kneading apparatus includes a plasticizing cylinder in which a high pressure kneading zone and a pressure reduction zone are formed adjacently in this order from an upstream side so that a molten resin obtained by plasticizing a thermoplastic resin and a pressurized fluid are kneaded with each other in the high pressure kneading zone, and gasified pressurized fluid, which is gasified by reducing a resin internal pressure, is separated in the pressure reduction zone from the molten resin kneaded with the pressurized fluid, a screw which is arranged rotatably and movably back and forth in the plasticizing cylinder; and a downstream side seal mechanism which is provided between the high pressure kneading zone and the pressure reduction zone and which makes communication and disconnection between the high pressure kneading zone and the pressure reduction zone in accordance with a rotation state of the screw.