Abstract: A tire forming drum includes a plurality of main segments arranged around a main shaft of the drum and rotatable together with the main shaft. The main segments are reciprocatingly radially movable toward and away from the main shaft to adjust the diameter of the drum and have circular arc surfaces, respectively, on which a tire constituent member is wrapped. The tire forming drum further includes subsegments having plural circular arc surfaces, respectively, to conform to change on diameter of the drum. The subsegments are arranged between the main segments to be movable to fill clearances occurring between the adjacent segments in response to the movements of the main segments.