PURPOSE: To improve the composite characteristics such as a tensile characteristic by hermetically sealing a multilayer reinforced ring structure into an annular can of special structure for HIP and subjecting the can to the HIP treatment.

CONSTITUTION: The ring structure 12 is constituted of a filament 14 for reinforcement and a matrix 16 in which the filament 14 is embedded. This multilayer reinforced ring structure 12 is enclosed in the inside of the can 10 for HIP. The can 10 is thin in an inner layer 18 and thick in an outer layer 20. This can 10 is welded to a normal position of the ring structure 12 to form an annular housing. Next, after evacuated, the can 10 is subjected to the HIP treatment for an appropriate length of time in the high temp. and high pressure atm. As a result of the compaction using this asymmetrical method, a reinforcing material of the filament 14 of a composite material is formed to a streched state, whereby the structure having the matrix 16 of the composite material kept in the compressed state is obtd. The ring structure is compacted preferentially from the inside, thus the bending and damage of the filament 14 for reinforcement at the outside section of the ring is avoided.