A plasticizing unit comprising: a heating cylinder ; a screw that is received rotatably inside the heating cylinder ; a resin supply hole and a fiber supply hole that are opened in the heating cylinder ; and a fiber supply device that supplies a fiber material into the heating cylinder through the fiber supply hole ; wherein: the fiber supply device includes at least one reel on which a long fiber material (A1) is wound, a cutting section by which the long fiber material pulled out from the reel is cut off into a prescribed length, and a pressure-feeding section by which fiber material pieces (A2) each cut in the prescribed length by the cutting section are pressed into the heating cylinder through the fiber supply hole ; the pressure-feeding section has a pressing cylinder that is attached to the heating cylinder so that one end of the pressing cylinder can communicate with the fiber supply hole , a pressing piston that is received slidably inside the pressing cylinder , a fiber supply pipe through which a head chamber of the pressing cylinder and the cutting section can communicate with each other, characterized in that the plasticizing unit comprises a vacuum device , that is coupled to an air exhaust pipe of a rod chamber of the pressing cylinder , for creating an air current through the rod chamber of the pressing cylinder , a gap between an inner surface of the pressing cylinder and an outer surface of the pressing piston and a head chamber of the pressing cylinder , which air current sucks the fiber material pieces (A2) staying in the cutting section and inside the fiber supply pipe into the head chamber of the pressing cylinder , and in that the gap is small such that the fiber material pieces (A2) cannot pass through the gap easily. the fiber supply device includes at least one reel on which a long fiber material (A1) is wound, a cutting section by which the long fiber material pulled out from the reel is cut off into a prescribed length, and a pressure-feeding section by which fiber material pieces (A2) each cut in the prescribed length by the cutting section are pressed into the heating cylinder through the fiber supply hole ; the pressure-feeding section has a pressing cylinder that is attached to the heating cylinder so that one end of the pressing cylinder can communicate with the fiber supply hole , a pressing piston that is received slidably inside the pressing cylinder , a fiber supply pipe through which a head chamber of the pressing cylinder and the cutting section can communicate with each other, characterized in that the plasticizing unit comprises a vacuum device , that is coupled to an air exhaust pipe of a rod chamber of the pressing cylinder , for creating an air current through the rod chamber of the pressing cylinder , a gap between an inner surface of the pressing cylinder and an outer surface of the pressing piston and a head chamber of the pressing cylinder , which air current sucks the fiber material pieces (A2) staying in the cutting section and inside the fiber supply pipe into the head chamber of the pressing cylinder , and in that the gap is small such that the fiber material pieces (A2) cannot pass through the gap easily. <EOS>