Abstract: A speed change device is provided, including: an outer ring part, an interior thereof including at least one outer gear; a spindle, coaxially pivoted to the outer ring part with the at least one outer gear, having a central axis, an outer circumference of the spindle having a plurality of cam assembling portions; a plurality of cam parts, respectively disassemblably assembled to the plurality of cam assembling portions, synchronizingly rotating with the spindle, at least two of the plurality of cam parts having a phase difference of angle which is larger than zero degree; a plurality of inner gears, being rigid, respectively sleeved on the plurality of cam parts and being non-rotatable with the plurality of the cam parts, respectively meshed with the at least one outer gear.