A roller device mountable rotatably on a spindle includes a roller body having a tubular wall with two opposite open-end portions. Each open-end portion is provided with a retaining member formed on an inner peripheral surface of the tubular wall. Two covers are inserted respectively into the open-end portions of the tubular wall, and are adapted to be sleeved rotatably around the spindle. Each cover has an annular end wall transverse to the corresponding open-end portion, a surrounding wall extending inwardly and axially from an outer periphery of the end wall and abutting against the inner peripheral surface of the tubular wall, and an engaging member formed on an outer peripheral surface of the surrounding wall and interlocking the retaining member. The retaining and engaging members have recess-and-protrusion interlocking surfaces.