A reduction gear device includes a first reduction mechanism and a second reduction mechanism. The second reduction mechanism includes a crankshaft, an external gear and an internal gear. One of the internal gear and the external gear is fixed to a base part side member of a robot arm of an industrial robot, and the other of the internal gear and the external gear is fixed to an anterior end side member of the robot arm of the industrial robot. An inner diameter D of the internal gear is 140 mm≦D≦200 mm, torque T transmitted to the anterior end side member is 650 Nm≦T≦1960 Nm, and the number of teeth N of the internal gear is less than 40 times the difference in the number of teeth between the internal gear and the external gear.