To facilitate the operation to recycle surplus resin and at the same time, obtain a hollow molding with the superior smoothness of an inner face by extruding a non-reinforced resin in the center of its layer, into an auxiliary cavity, leaving the rest of the non-reinforced resin forming an inner layer as it is on the inner peripheral face of a reinforced resin forming an outer layer.

A receiving shaft 7 is slightly moved backward to such an extent that a floating core 2 can be accepted into an auxiliary cavity 6, and a communication aperture 5 is opened. At the same time, a pressure fluid is injected under pressure into a pressure port. Consequently, the floating core 2 heads for the auxiliary cavity 6, extruding the slow-cooling molten resin (almost non-reinforced resin) in the center of the non-reinforced resin into the auxiliary cavity 6 through the communication aperture 5. During this process, a reinforced resin which begins to be solidified by cooling on the outer peripheral side of a main cavity is left behind with the remaining non-reinforced resin of some thickness on the inner face side. In the wake of the floating core 2 passing by, a hollow part 11 with an almost equal diameter to the diameter of the floating core 2 is formed.