PURPOSE: To provide a universal small-volumetric differential roller speed reducer and a geared motor.

CONSTITUTION: A shaft of a motor M is utilized as an input shaft 4, attaching a fixed cylinder 2 to the shaft side of a motor bracket 1 concentrically with the input shaft 4, and an idler cylinder 5 is installed at a root side of the input shaft 4 free of rotation, while a first idler roller 6 is interred between the idler cylinder 5 and an inner surface of the fixed cylinder 2. In addition, a second idler roller 14 concentrically clamped and connected to a tip side of the first idler roller 6 is made contact with the tip side so as to receive turning effort, and an output cylinder 16 is put on an outer side of the second idler roller 14 so as to be rolled, and then a speed reducer outputting a difference between rotation and revolution of the second idle roller 14 and the motor M are built in together. With this constitution, the occurrence of noise is well checkable as compared with a gear type speed reducer and thus miniaturization is thus promoted and, what is more, it is usable in terms of general purpose.