A speed changer structure, comprising: a transmitting mechanism , which comprises a fixed shaft , an output frame , and a toothed ring , wherein a primary sun gear is mounted by means of a one-way bearing to the fixed shaft , and the output frame is rotatably mounted to an end of the fixed shaft , the output frame comprising at least two bars that are parallel to the fixed shaft and jointly define a circle having a diameter that is smaller than an inside diameter of the toothed ring , each of the bars of the output frame being provided with a primary planetary gear rotatably mounted thereto and in mesh engagement with an inner circumference of the toothed ring , the toothed ring having an outer circumference in mesh engagement with a driving wheel ; and at least one speed-increasing mechanism , which comprises a tubular shaft , a sun gear , at least two planetary gears , and an arresting assembly , wherein the tubular shaft is rotatably mounted to the fixed shaft , and the sun gear is mounted to a first end of the tubular shaft , the sun gear having an outside diameter that is smaller than an outside diameter of the primary sun gear , the planetary gears being respectively and rotatably mounted to the bars of the output frame , the planetary gears being respectively combined with the primary planetary gears to be synchronously rotatable with each other, the planetary gears being in mesh engagement with the sun gear , the arresting assembly being mounted to a second end of the tubular shaft to selectively stop rotation of the tubular shaft ; wherein the driving wheel is driven by a power source and is in mesh engagement with the outer circumference of the toothed ring of the transmitting mechanism to generate an output at the output frame , and an effect of increasing speed is achievable through the arresting assembly of the speed-increasing mechanism fixing the tubular shaft . a transmitting mechanism , which comprises a fixed shaft , an output frame , and a toothed ring , wherein a primary sun gear is mounted by means of a one-way bearing to the fixed shaft , and the output frame is rotatably mounted to an end of the fixed shaft , the output frame comprising at least two bars that are parallel to the fixed shaft and jointly define a circle having a diameter that is smaller than an inside diameter of the toothed ring , each of the bars of the output frame being provided with a primary planetary gear rotatably mounted thereto and in mesh engagement with an inner circumference of the toothed ring , the toothed ring having an outer circumference in mesh engagement with a driving wheel ; and at least one speed-increasing mechanism , which comprises a tubular shaft , a sun gear , at least two planetary gears , and an arresting assembly , wherein the tubular shaft is rotatably mounted to the fixed shaft , and the sun gear is mounted to a first end of the tubular shaft , the sun gear having an outside diameter that is smaller than an outside diameter of the primary sun gear , the planetary gears being respectively and rotatably mounted to the bars of the output frame , the planetary gears being respectively combined with the primary planetary gears to be synchronously rotatable with each other, the planetary gears being in mesh engagement with the sun gear , the arresting assembly being mounted to a second end of the tubular shaft to selectively stop rotation of the tubular shaft ; wherein the driving wheel is driven by a power source and is in mesh engagement with the outer circumference of the toothed ring of the transmitting mechanism to generate an output at the output frame , and an effect of increasing speed is achievable through the arresting assembly of the speed-increasing mechanism fixing the tubular shaft . <EOS>