PURPOSE: To prevent an additive material from scattering defectively, by a method wherein a high shearing kneading part is provided on a position more outlet side than a casting position of the additive material of an extruding machine in the title device where a plurality of units of extruding machines are connected in order with each other and kneading is performed by adding the additive material within the extruding machine on and after the second stage.

CONSTITUTION: In the first stage extruding machine 1, a pellet of thermoplastic resin fed within a cylinder 2 through a hopper 3 is kneaded by a screw 4 along with an aging preventive agent as an additive agent 5 having the high melting point while heating and melting the pellet and conveyed to an outlet side. Then after removal of a fine foreign matter with a screen pack 8 and breaker plate 9, crosslinking agent as an additive agent 13 is cast into a cylinder 12 with a pump 15 through a holding vessel 14 during a process where conveying is performed while kneading by a screw 16 in the second stage extruding machine 11. In this case, a high shearing kneading part 20 performing kneading by the higher shearing force than that of a previous stage side of the screw 16 is provided on a position more an outlet side than a casting position of the additive agent 13 and the additive agent 13 is scattered and mixed up with each other uniformly.