PURPOSE:To eliminate the need for using a bearing on a rotation transmitting part and simplify the whole structure by forming an annular rolling ball driving face on the outer peripheral face of the end part of an input shaft and carrying out speed change by means of the friction power transmission of a ring rolling ball in between said driving face and a rolling ball pressing face on the output side. CONSTITUTION:A pair of guide rings 4, 5 are adjacently provided on the inner peripheral face side, on the center part in the axial direction of a cylindrical casing 1, and plural engaging parts 61-63, 71-73 are formed at an equal interval on the outer peripheral parts on the mutually opposite sides of the rings 4, 5 respectively. Ar!d, engaging balls 81-83 which are engaged with the engaging recessed parts 91-93 formed on the inner peripheral face of the casing 1 are engaged with the engaging parts respect- tively. Also, a rolling ball driving part 15 having a pair of rolling ball driving faces 16, 17 which are brought into contact with three rolling balls 121-123, 131-133 respectively, is formed on the end part of an input shaft 2. And, pressing rings 23, 27 which are energized by springs are provided displaceably along torque transmitting rods 191-193, in the mutually opposite positions interposing the rolling ball driving part 15 and the rolling balls 121-123, 131-133.