PURPOSE: To use scraps directly as an HMW-PE (ultra-high density polyethylene of ultra-high molecular weight) raw material without pelletizing flashes and the like by kneading the scraps to be generated through a continuous kneader in case one resin layer of two resin layers not fused each other is thinner than the other resin layer and continuously feeding the scraps in the molten state into a melted portion in a resin extruder forming a thick layer.

CONSTITUTION: Flashes and the like melted and kneaded by a two-runner kneading extruder 11 are fed directly into a base section of a molten portion in an HMW-PE extruder 1, mixed with the HMW-PE and fed into a die 4 to form an HMW-PE layer of a parison 5. At that time, as nylon 6 contained in the flashes and the like is far less than the HME-PE, and said nylon is further diluted and contained in the state of fine particles, the strength of a tank is not lowered at all.