Apparatus for forming an annular product features in such arrangement that, around a drum shaft sheathing over a main shaft, tapering structures each having axial slidability are provided, and in response to a slide move of the tapering structure, drum segments are each radially expanded or retracted. A stopper for controlling a slide move of the tapering structure is provided and an axial position of the stopper is regulated by a motor through a ball screw mechanism disposed in the main shaft. Thereby, settings and changes of a circumferential length of the drum which determines the length of the annular product are easily made and can be made with great certainty.