A belt drum device for forming a belt structure including a belt to be used in a tread of a tire comprising: a drum base rotatable around a drum rotational axis , a plurality of belt support portions arranged outside the drum base around the drum rotational axis in a drum circumferential direction, and a plurality of expansion and contraction means each for expanding and contracting corresponding one of the belt support portions in a drum radial direction, wherein each of the plurality of the belt support portions comprises a first link plate of which outer end is rotatably connected with a first end of the drum base , a second link plate of which outer end is rotatably connected with a second end of the drum base , and a pin connecting inner ends of the first link plate and the second link plate so as to be movable in the expanding and contracting direction, each of the first link plate and the second link plate has an outer peripheral surface around which the belt is wound in the drum circumferential direction, in a transverse cross-sectional view passing through the drum rotational axis of the first link plate and the second link plate in a contracted state, each of the outer peripheral surfaces comprises a flat surface portion extending straight on a side of the outer end in a drum axial direction and an arc-shaped curved surface portion connected with the flat surface portion and curved toward the drum rotational axis as it approaches the inner end , in a transverse cross-sectional view passing through the drum rotational axis of the first link plate and the second link plate in an expanded state, the curved surface portion of the first link plate and the curved surface portion of the second link plate are substantially continuous so as to form a continuous arc region convex outwardly in the drum radial direction, and the continuous arc region in the expanded state has a width (WB) in a range of from 30% to 85% of a width of the outer peripheral surface . a drum base rotatable around a drum rotational axis , a plurality of belt support portions arranged outside the drum base around the drum rotational axis in a drum circumferential direction, and a plurality of expansion and contraction means each for expanding and contracting corresponding one of the belt support portions in a drum radial direction, wherein each of the plurality of the belt support portions comprises a first link plate of which outer end is rotatably connected with a first end of the drum base , a second link plate of which outer end is rotatably connected with a second end of the drum base , and a pin connecting inner ends of the first link plate and the second link plate so as to be movable in the expanding and contracting direction, each of the first link plate and the second link plate has an outer peripheral surface around which the belt is wound in the drum circumferential direction, in a transverse cross-sectional view passing through the drum rotational axis of the first link plate and the second link plate in a contracted state, each of the outer peripheral surfaces comprises a flat surface portion extending straight on a side of the outer end in a drum axial direction and an arc-shaped curved surface portion connected with the flat surface portion and curved toward the drum rotational axis as it approaches the inner end , in a transverse cross-sectional view passing through the drum rotational axis of the first link plate and the second link plate in an expanded state, the curved surface portion of the first link plate and the curved surface portion of the second link plate are substantially continuous so as to form a continuous arc region convex outwardly in the drum radial direction, and the continuous arc region in the expanded state has a width (WB) in a range of from 30% to 85% of a width of the outer peripheral surface . <EOS>