PURPOSE: To remove the entrained air in a cylindrical composite rubber preform, by inserting a mandrel into the cylindrical composite rubber preform, fitting a resilient tube on the preform, winding a tape round the resilient tube, and heating them to a temperature at which vulcanization would not occur.

CONSTITUTION: The mandrel 19 is inserted into the composite rubber preform 18 in which, as reinforcing layers for an inner unvulcanized rubber sheet 14, cord fabric cords 15, 16 wound in opposite directions and an outer unvulcanized rubber sheet 17 are wound. The buffering resilient tube 20 having an inner diameter smaller than the outer diameter of the preform 18 is fitted on the preform 18. The holding tape 21 is wound round the tube 20, and the assembly is introduced in a temperature constant bath, where it is heated at a temperature at which vulcanization would not proceed.