PURPOSE: To provide a reduction gear which is small, low-costed and of low noise even with a high reduction gear ratio available.

CONSTITUTION: A planetary reduction gear set has an input shaft 11; an eccentric shaft 12 secured to the input shaft 11; a planetary gear 13a freely rotatably supported to and concentric with the eccentric shaft 12; a fixed internal gear 14 constantly meshing at part with the planetary gear 13a and differing in the number of teeth with the planetary gear 13a; and an output shaft 16a. A steel ball 15 is assembled between an end face of the boss 13b of the planetary gear 13a and a circular recessed portion in an end face of the output shaft so as to take out only an output of decelerated rotation of the planetary gear 13a on its axis without preventing the planetary gear 13a from rotating around the shaft 12.