To efficiently assemble or manufacture a belt by fastening a plurality of elements in a circular manner using a ring. A belt assembling device is adapted to assemble a belt by fastening a plurality of plate-like elements juxtaposed in a same orientation and in a circular manner using a ring. The belt assembling device includes a holding member. The holding member holds the ring to which the elements are attached sequentially, in a manner to keep a curvature radius of the ring partially larger than that of the remaining portion. Therefore, the portion of the ring at which the curvature radius is thus kept larger can be twisted and deformed easily so that the element can be attached to the ring at the portion.