The invention provides a stepless speed change device which comprises a shell body, a left semisphere shaft, a right semisphere shaft, a transmission shaft, a transmission shaft support, a direction rotating shaft and the like. The transmission shaft has the same profile curve as the left semisphere shaft and the right semisphere shaft, and therefore the transmission shaft, the left semisphere shaft and the right semisphere shaft throughout keep line contact in a movement process. Power is input from the left semisphere shaft, and the transmission shaft rotates around the transmission shaft under the action of friction, and drives the right semisphere shaft to rotate to achieve power output. The transmission shaft support is driven by the direction rotating shaft to rotate by a certain angle in space along with the direction rotating shaft, the contact radius of a left end and the contact radius of a right end of the transmission shaft are enabled to change, and therefore a change of a transmission ratio is achieved. When the direction rotating shaft continuously rotates in the space, continuous changes of the transmission ratio are achieved, and stepless speed adjustment is achieved at an output end of the right semisphere shaft. The stepless speed change device has the advantages of being simple in whole structure, convenient to manufacture, capable of being installed in small working space, and capable of changing power which is capable of being transmitted by changing the pressure of the transmission of the transmission shaft, the pressure of the left semisphere shaft, the pressure the right semisphere shaft, the length of a contact curve between the transmission shaft and the left semisphere shaft and the length of a contact curve between the transmission shaft and the right semisphere shaft, and satisfying various requirements of the power.