To provide a planetary reduction gear starter, protected from damage caused by an abrupt change in rotating torque, even when a load to be started is increased abruptly.

A starter has a planetary reduction gear device, so that the rotating output from a motor is decelerated and transmitted to a load to be started. The motor has an armature 3, a motor output shaft 31 and a cylindrical yoke 1. The planetary reduction gear device has an internal gear 8 fitted at an end of the yoke 1, an inner circumference gear 11 in the internal gear 8, a bearing 13 at a central bottom part of the internal gear 8, an outer circumference gear member 32 at the motor output shaft 31, a planetary gear 4, provided rotatably between the internal gear 8 and the inner circumference gear member 11, and a starting output shaft 21 pivotally supported by the bearing 13. The planetary gear 4 is supported pivotally by the output shaft 21 on one end, while the output shaft 21 drives the load to be started on the other side. In addition, an engagement part 9 between one end of the yoke 1 and the internal gear 8 is slid in a circumferential direction, when a rotating torque greater than a given level is applied.