PURPOSE: To provide a magnet style differential type ball planetary reduction gear capable of easily maintaining and controlling a frictional force of steel balls, simplifying the structure and facilitating the change of reduction ratio.

CONSTITUTION: A steel-made drive disk 2 is fixed to a drive shaft 1 and first and second magnets 3, 4 are fixed to both sides of the disk. Rows of first and second steel balls 6, 8 are annularly disposed on both sides of the disk 2 while being held by retainer 7 to be simultaneously revolved. The drive shaft 1 is supported by a fixing member 9 so that a driven shaft 13 is supported relatively rotatably on the axis common to the drive shaft. The fixing member 9 is provided with a bearing tube 11, a regulating ring 12, an outer contact ring 13 and an inner contact ring 14. A steel-made driven disk 14 rollingly touching a row of steel balls 8 is fixed to a shaft 13. The reduction ratio is changed by the movement of a contact point toward the steel ball 6 accompanying the relative axial movement of the inner contact ring 14 and the outer contact ring 13.