A decelerating device includes a fixed shaft, a tube, a transmission element and a driven element is provided. The tube is disposed around the fixed shaft and adapted to rotate about the fixed shaft. The transmission element has a fixed portion fixed at the fixed shaft and a planetary gear revolvably disposed at the fixed portion. The teeth of the tube are geared to the planetary gear. The driven element has a wheel, a sun gear fixed at the wheel and a plurality of balls. The sun gear and the wheel are revolvably disposed at the fixed shaft and in the tube. The planetary gear is adapted to drive the sun gear. The wheel has holes. Each of the holes extends from an outer surface of the wheel facing the tube to the interior of the wheel to form a cavity. The balls are disposed in the holes respectively.