PURPOSE: To provide a ball planetary speed reducing device in a magnet type differential type which can easily maintain and control the frictional force of a steel ball and simplify structure and reduce dimension.

CONSTITUTION: A driving disc 2 made of iron is fixedly installed on a driving shaft 1, and on both the sides of the driving disc 2, the first and second annular magnets 3 and 4 are fixed, setting the same poles opposed in the axis line direction. On both the sides of the driving disc 2, the first and second steel ball rows 6 and 8 are arranged in an annular form, and held by a retainer 7 so as to revolve simultaneously around each axis. The driving shaft 1 is supported on a fixed member 9, and a driven shaft 13 is supported in a relatively turnable manner on the common axis center to the driving shaft 1. On the driven shaft 13, a driven disc 14 which is made of iron and contacts the second steel ball 8 row in free rolling, opposed to the driving disc 2, is fixed. On the steel ball rolling surfaces of the fixed member 9 and the driven disc 14, the annular recessed places 12 and 20 in which the first and second steel ball rows 6 and 8 drop and roll on the edge parts are formed.