This gear speed change device is provided with an input unit 13 in which an eccentric guide portion 14 is formed centered about an eccentric position x1, on an input shaft 11 having an axial center x, an internal/external gear 20 which has external teeth 18 and internal teeth 19 having the center of rotation at the eccentric position x1, and one side of which is rotatably held by the eccentric guide portion 14, an output unit 24 which has an output shaft 12 on the axis x and on one side of which are formed internal teeth 22 that mesh with the external teeth 18 and that are centered about the axis x, and a rotation control unit 25 for controlling the rotational speed of the output unit 24.