To provide a planetary differential type motion converting mechanism suppressing the occurrence of uneven wear or fracture on the tooth faces of a screw and a gear formed on a planetary shaft and of a screw and a gear meshing with the screw and the gear, resulting from the inclination of the planetary shaft.

The planetary differential type motion converting mechanism 100 includes a cover 40 fixed to the inner peripheral face of a rotor 10 for restricting the axial displacement of a rear-side planetary gear 32b. The cover 40 includes a circular groove 41 formed in a position opposite to a bearing hole 34 of the rear-side planetary gear 32b. To the groove 41, a ball 42 is fitted which is slidably fitted to the bearing hole 34 of the rear-side planetary gear 32b to support the planetary shaft 30 at its rear-side end.