A speed changer includes a transmitting mechanism and at least one speed-increasing mechanism. The transmitting mechanism includes a primary sun gear mounted by a one-way bearing a fixed shaft. The primary sun gear is in mesh engagement with primary planetary gears rotatably mounted to an output frame and in mesh engagement with an inner circumference of a toothed ring. The speed-increasing mechanism is rotatably mounted by a tubular shaft to the fixed shaft. The tubular shaft is provided with a sun gear having a diameter smaller than the primary sun gear. Planetary gears in mesh engagement with the sun gear are rotatably mounted on the output frame and are combined with and synchronously rotatable with the primary planetary gears. The tubular shaft of the speed-increasing mechanism is provided with an arresting assembly to selectively stop rotation of the tubular shaft.