PURPOSE: To obtain a helmet having three-layer structure or sandwich structure, by a method wherein a composite material, consisting of a second synthetic resin and an adder, is injected into a cavity from a second injection molding cylinder while continuing the injection of a first synthetic resin from a first injection molding cylinder.

CONSTITUTION: Single synthetic resin A, supplied into a cylinder 2, is injected into a cavity 16 through a sprue 15. When a predetermined amount is achieved, an auxiliary cylinder 8 is operated to open a needle valve 12, then, composite material B is supplied by an auxiliary screw 14 into a cylinder head 4 through a connecting duct 13. The composite material B is mixed by a mixing nozzle 6 with the synthetic resin A, being supplied continuously into the cylinder head 4, then, is injected into the cavity 16 through the sprue 15. According to this method, the single synthetic resin A, injected into the cavity 16, forms the skin layer of an injected form while the mixture of the single synthetic resin A and the composite material B creeps into uncured single synthetic resin A and forms the core layer of the injected form.