It is intended to provide a technique for making a biomass-origin dispersion phase of a polymer composite material, wherein the dispersion phase is formed in a synthetic polymer matrix, highly microscopic and homogeneous. An apparatus (10) for producing a polymer composite material characterized by comprising: a cylinder (13) in which a supply area (A) for supplying the main component of the synthetic polymer as described above and an excessive water-containing component originating in a biomass as described above is provided in the upstream side while a discharge unit (17) is provided in the downstream side; a screw (15) which axially rotates within the cylinder (13), which has been set to a kneading temperature (Tz) at which the synthetic polymer melts, and extrudes the kneaded matter comprising the main component and the excessive water-containing matter as described above toward the discharge unit (17); and a dehydration unit (40) by which the water contained in the kneaded matter is removed under setting pressures (Pa, Pb, Pc) being lower than the saturated vapor pressure (Pz) at the kneading temperature (Tz) employed in the cylinder (13) but higher than atmospheric pressure.