A speed reducer having rollers axially longer than its cage, includes coaxial input and output shafts with mutualy facing ends covered by a housing that supports an internal gear. An eccentric disk is mounted on an end of the input shaft to be located radially inwardly of, and rotatable relative to, the internal gear. A cage at the end of the output shaft is between the internal gear and a bearing fitted on the disk. The cage has pockets unequal in number to internal teeth of the internal gear. A roller in each pocket is rollable along the radially outer surface of the bearing outer race. The input shaft rotation is transmitted to the shaft at a reduced speed ratio determined by the number of the internal teeth. An annular recess is formed along the corner between the closed end and radially inner surfaces of the cage.