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 (20, 30), which comprises a tubular shaft (21, 31), a sun gear (23, 32), at least two planetary gears (24, 33), and an arresting assembly (25, 35), wherein the tubular shaft (21, 31) is rotatably mounted to the fixed shaft, and the sun gear (23, 32) is mounted to a first end of the tubular shaft (21, 31), the sun gear (23, 32) having an outside diameter that is smaller than an outside diameter of the primary sun gear, the planetary gears (24, 33) being respectively and rotatably mounted to the bars of the output frame, the planetary gears (24, 33) being respectively combined with the primary planetary gears to be synchronously rotatable with each other, the planetary gears (24, 33) being in mesh engagement with the sun gear (23, 32), the arresting assembly (25, 35) being mounted to a second end of the tubular shaft (21, 31) to selectively stop rotation of the tubular shaft (21, 31); 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 (25, 35) of the speed-increasing mechanism (20, 30) fixing the tubular shaft (21, 31). 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 (20, 30), which comprises a tubular shaft (21, 31), a sun gear (23, 32), at least two planetary gears (24, 33), and an arresting assembly (25, 35), wherein the tubular shaft (21, 31) is rotatably mounted to the fixed shaft, and the sun gear (23, 32) is mounted to a first end of the tubular shaft (21, 31), the sun gear (23, 32) having an outside diameter that is smaller than an outside diameter of the primary sun gear, the planetary gears (24, 33) being respectively and rotatably mounted to the bars of the output frame, the planetary gears (24, 33) being respectively combined with the primary planetary gears to be synchronously rotatable with each other, the planetary gears (24, 33) being in mesh engagement with the sun gear (23, 32), the arresting assembly (25, 35) being mounted to a second end of the tubular shaft (21, 31) to selectively stop rotation of the tubular shaft (21, 31); 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 (25, 35) of the speed-increasing mechanism (20, 30) fixing the tubular shaft (21, 31).