PURPOSE: To prevent generation of a sink in the surface and a vacuum foam on the inside of an end part, by a method wherein after an opening part of a molding main body is coated with a molding material on the end part and a fluid is pressed into the molding material through a hollow part of the molding main body before plasticity of the molding material is lost.

CONSTITUTION: After an opening part 38 of a molding main body part 31 is coated with a molding material 61, before curing of the molding material, that is, before plasticity is lost, high-pressure gas is blown off through a nozzle 57 and the high-pressure gas is pressed into the molding material 61 through a hollow part 37 or an opening part 38 of the molding main body part 31. The molding material 61 is adapted itself to a cavity surface by expanding the molding material 61 and when the molding material 61 is cured under a state where inner pressure is kept high, since the molding material is pressed always against the cavity surface 47a as a whole even if capacity of the material itself is contracted, it become that a sink is hardly generated. In the case where the sink is generated, it is generated into the surface of a hollow part 39a at the end part 39. Similarly to the above, a vacuum foam also is absorbed into the hollow part 39a.