A roller device for attaching to a shaft of a treadmill, includes a tubular housing having two carriers engaged in the end portions, and the carriers each includes a casing directly folded from the end portion of the housing, and two bearing members are engaged in the casings of the carriers, for engaging with the shaft, and for smoothly and rotatably attaching the housing to the shaft, and for allowing the roller device to be easily and readily attached to treadmills. The casing includes an outer peripheral flap formed integral with the housing, and includes an outer diameter smaller than an inner diameter of the housing, for resiliently supporting the casing within the housing.