To reduce the unnecessary working, shorten the working time, and reduce the manufacturing cost by correcting the roundness of a ring material by a hot press of the recrystallizing temperature or more to improve the roundness and produce precision.

A ring material 10e formed into a uniform section is heated 580°C in an induction furnace, and baked at high speed to lay the material tissue of a weld part into ordinary state. The baked ring material 10f is set so as to be nipped between a lower metal mold 17c and an upper metal mold 17d assembled to the bed 17a and ram 17b of a press machine while it keeps a constant temperature not lower than the recrystallizing temperature (about 450°C), and a corrugated elliptic state ring material 10f is extended by pressing by adding a load less than 100 tons, and molded and corrected so as to form a circular ring. The process from baking to threading can be simplified, and the working time can be shortened.