A reduction gear transmission in which a plurality of reduction units is combined to realize a large torque, without increasing the diameter of a second reduction gear unit, and in which a through hole can be maintained is taught. A first reduction gear unit 34 has a first input shaft 24 that rotates in accordance with the rotation of an output shaft 29 of a motor 28, a first external gear that revolves orbitally around an axis line of the first input shaft 24, and a first internal gear 36 that has a number of teeth differing from the number of teeth of the first external gear. A second reduction gear unit 2 comprises a crankshaft 122 that rotates in accordance with the rotation of a first output shaft 20, second external gears 162X and 162Y having a through hole formed at a center thereof, a cylindrical member 16 passing through the through holes of the second external gears 162X and 162Y, a second internal gear 4 that has a number of teeth differing from the number of teeth of the second external gears 162X and 162Y, and a second output shaft that rotates around an axis line of the second internal gear 4. The first reduction gear unit 34 is disposed between an outer circumferential surface 18 of the second reduction gear unit 2 and an inner surface 14 of the cylindrical member 16.