To provide a reduction gear which is easily fabricated and reduced in cost.

The reduction gear comprises: a fixed-side internal contact surface 23 circumferentially formed in a fixed-side housing 3; a flexible ring 5 which is formed to have a circumferential length shorter than the fixed-side internal contact surface 23 and is arranged so as to envelope the fixed-side internal contact surface 23, and partially come into contact with the fixed-side internal contact surface 23 in the circumferential direction by radial bending so as to change the contact position; a plurality of balls 7 inscribing the flexible ring 5, and changing the inscribed position so as to change the contact position of the flexible ring 5; and a sun ring 9 circumscribing each ball 7 from the inner peripheral side of the ring-like arrangement and transmitting the input rotation of an input shaft 45 to each ball 7.