To continuously and properly perform desired molding by a simple technique and means without being accompanied by the clogging of a resin passage with a resin of which the thermal physical properties are unstable.

When an outer layer resin is accumulated in a die outside accumulation chamber 25, a valve 30 is throttled and the plunger P1 in the die outside accumulation chamber 25 is allowed to retreat and, after the retreat of the plunger P1 is completed, the number of rotations of the screw of the corresponding extruder 28 is increased to reduce the amount of the resin flowing in the die outside accumulation chamber 25 and an outside resin passage 22 and the inner pressure of an inside resin passage 20 is lowered, for example, by moving the plunger of a die inside accumulation chamber 23 and the outer layer resin present in the outside resin passage 22 is allowed to flow in an intermediate resin passage 21 or the inside resin passage 20 by the pressure difference between the resin passages 20, 22 to eliminate the stagnation of the resin in the resin passage 22.