A stepless speed change device has an input shaft and an output shaft. The device inlcudes a planetary carrier proviced at an inner end of the input shaft, a sun gear provided at an inner end of the output shaft to be in mesh with planet gears carried on the planetary carrier, and an internal gear in mesh with the planet gears to form a planetary gear. The device further includes a driven rotor rotatable about the input shaft and in the form of a hollow cylinder having flanges at both ends and concentrically connected to the internal gear, a driving rotor enclosing the driven rotor and including two members connected to the input shaft on both sides of the driven rotor, and an intermediate friction transmission ring whose eccentricity is adjustable and interposed between the flanges of the driven rotor and between the two members of the driving rotor for transmitting rotating power therebetween.