In order to miniaturize a speed change gear assembly, the respective gears are arranged in the order of the first gear A.sub.1, the second gear A.sub.2, the fourth gear A.sub.4 and the third gear A.sub.3 in an axial direction (X-axis direction). A center of the common spherical surface passing through each pitch circle of the first and second gears is made in accordance with the center of the common spherical surface passing through each pitch circle of the fourth and third gears. The center is used as the origin O. An XY coordinate is provided on the origin O. An axis line of an input shaft is arranged on the X-axis of the XY coordinate. An engagement point between the first and second gears A.sub.1 and A.sub.2 and an engagement point of the fourth and third gears A.sub.3 and A.sub.4 are arranged in the same quadrant of the XY coordinate. The first to fourth gears are arranged on the same axial direction surface of a rotary member 3. It is therefore possible to thereby reduce the axial directional dimension of the gear assembly.