A planetary gear system has a driven sun gear, an internal gear fixed on a case concentrically with the sun gear, a rotatable shaft concentric with the sun gear having a carrier, and planetary gears on the carrier for engagement with the sun gear and the internal gear. The internal gear is divided in the axial direction into first and second internal gear sections which are adjustable in their relative angle in the circumferential direction so as to reduce a backlash in the engagement with the planetary gears. The planetary gears comprise a large planetary gear part for engagement with the sun gear and a small planetary gear part concentric with the large planetary gear part for engagement with the internal gear.