An eccentrically oscillating type reduction gear includes an internal gear, an external gear which meshes with the internal gear, and an eccentric body which oscillates the external gear. The external gear is formed of a resin, and the internal gear has an internal gear main body which is formed of a resin, and an outer pin which is rotatably disposed in a pin groove provided in the internal gear main body and is formed of a material having a thermal conductivity higher than that of the resin constituting the internal gear main body.