A reduction gear includes a reduction mechanism, which includes: an external gear eccentrically rotatable about an input shaft; an internal gear internally meshing with the external gear and having a small number of difference between the teeth of the internal gear and the external gear; a disk for extracting only a rotational component of the external gear; an output shaft; and gears for transmitting the rotational component of the external gear from the disk to the output shaft. The disk, the gears and the output shaft are aligned in a direction perpendicular to an axial direction of the input shaft.