PURPOSE: To omit predrying process prior to manufacture of strengthened polyester resin, by kneading undried thermoplastic aromatic polyester, a reinforcing agent and the other additives under specific conditions by making use of a special extruding machine.

CONSTITUTION: An extruder possesses two material supply ports and two vent holes, which are provided in order of the supply port 21, the vent hole 22, the supply port 23 and the vent hole 24. As for the extruder, thermoplastic aromatic polyester is supplied from the supply port 21 and a reinforcing agent and the other additives, for example, a filler, a core material, an accelerator of crystallization, a fire-resistant agent, a coloring agent and a plasticizer are supplied from the supply port 23. A staying time of polymer from the supply port 21 to the vent hole 22 is less than 120sec which is desirably less than 60sec, and the same from the port 23 to the vent hole 24 is less than 80sec which is desirably less than 50sec. The vent holes 22, 24 are connected with a vacuum generator, and at the time of kneading the respective vent holes are kept under decompressed states at less than 100 Torr, desirably less than 50 Torr.